I 











Glass _QJCl 
Book— i_v6>. 



^£j 



POPULAR TREATISE 



VEGETABLE PHYSIOLOGY. 



PUBLISHED UNDER THE AUSPICES OF THE SOCIETY FOR THE PROMOTION 
OF POPULAR INSTRUCTION. 



WITH NUMEROUS CUTS. 



PHILADELPHIA : 

LEA & BLANCHARD. 
1842. 



-■ * * * •■ . 






GRIGGS to CO., PRINTERS. 



WIT&ftfcAVvN 
MAY 9 1919 

PUB i3 LIMRART 
V- ASH NOTON, - D. C. 



CONTENTS. 



INTRODUCTION, - 

CHAPTER I. 

OP THE GENERAL CHARACTERS OF LIVING BEINGS, AND THE 
DISTINCTION BETWEEN ANIMALS AND VEGETABLES, - 

CHAPTER II. 

GENERAL VIEW OF THE VEGETABLE KINGDOM, - 

CHAPTER III. 

OF THE ELEMENTARY STRUCTURE OF PLANTS, 

CHAPTER IV. 

STRUCTURE AND FUNCTIONS OF THE ROOTS, - 

CHAPTER V. 

OF THE STRUCTURE AND FUNCTIONS OF THE STEM, - 

CHAPTER VI. 

OF THE FOOD OF PLANTS, AND THE MANNER IN WHICH IT 



IS OBTAINED, 



rftibllc i 
RECEr 
MAY i 9 




Page. 
9 



17 



23 



61 



81 



94 



11-2 



VU1 CONTENTS. 

CHAPTER VII. i 

> 

ON THE STRUCTURE OF LEAVES, - - - - 144 

CHAPTER Vffl 

OP THE FUNCTIONS OF THE LEAVES, - - - - - 164 

CHAPTER IX. 

GENERAL REVIEW OF THE NUTRITIVE POWERS IN PLANTS, - 198 

CHAPTER X. 

OF THE SECRETIONS OF PLANTS, - 220 

CHAPTER XL 

OF THE PRODUCTION OF LIGHT, HEAT AND ELECTRICITY BY 

PLANTS MOTIONS OF PLANTS, - - - 254 

CHAPTER XII. 

OF THE REPRODUCTION OF PLANTS, - • * - - 264 



ADVERTISEMENT. 



This volume was published in London under the auspices 
of the " Society for the Promotion of Popular Instruction " 
and the American publishers have much pleasure in offering it 
to the public. " Feeling assured that it will be found sufficiently 
simple in its character, and clear in its explanations, to be re- 
garded as an elementary treatise, adapted to those who have no 
previous knowledge of the subject; whilst its systematic arrange- 
ment, and the scientific value of the principles laid down in it, 
render it an excellent introduction to more comprehensive works 
on the same subject. The general reader, who seeks no more 
than entertainment or recreation, will find it in this volume, in 
the copious illustrative facts and interesting collateral informa- 
tion, with which it abounds; whilst to the Agriculturist, the 
Gardener, and the Domestic Economist, it supplies principles 
and practical applications of great importance." 

Philadelphia, May, 1842. 



INTRODUCTION. 



Of all departments of Science there is perhaps no single one 
capable of exercising such an advantageous influence on the mind 
of its cultivator as Natural History. Every kind of knowledge 
has in it something that is valuable; for even if it be of no direct 
utility in the ordinary concerns of the world, the acquirement of 
it is a useful exercise to the mental faculties, and the possession 
of it may operate in a most beneficial manner on the habitual 
feelings, and give a corresponding direction to the whole course 
of life. 

It is desirable to cherish correct views of the benefits of different 
kinds of knowledge, that those may choose most advantageously 
for themselves, whom the necessary business of life debars from 
the extended pursuit of it; and without undervaluing other 
branches of Science, it may be safely affirmed that Natural His- 
tory is capable of affording more to interest and instruct, more to 
refresh and relax the well-disposed mind on a very slight acquaint- 
ance with it, than any other pursuit. Not a step can the learner 
advance in it but he meets with wonders previously unsuspected; — 
not a height does he gain, from which his prospect is clearer and 
more extensive, but his notion of these wonders acquires a yet 
more astonishing vastness. The more he knows, the more he 
desires to know ; and the farther he advances, the more does he 
perceive how much delight is yet in store for him. 

The beneficent Creator of all has not only ordained that every 
part of his works should be good, — should be adapted to answer 
its designed end, and should contribute in highest degree of 
2 



10 INTRODUCTION. 

which it is capable to the well-being of his creatures ; — but he 
has made every thing " beautiful in its season," — he has so formed 
the mind of man that it derives pleasure from the contemplation 
of the glorious works around him. And it is, therefore, a worthy 
employment of our faculties to encourage this pleasure, and to 
place it upon a more solid and extended foundation than that 
afforded by the mere forms and colours of the objects around us, 
however beautiful these may be. One great source of the pleasure 
derived from the inquiry into the structure and mode of exist- 
ence of the living beings around us, arises from the beautiful 
adaptation of their parts to each other, and of the whole to the 
place it has to occupy, which we can easily trace in every 
one. The Philosopher who studies the motions of the heavenly 
bodies, and the station of this earth among them, traces these 
adaptations no less clearly ; but it requires profound and long-con- 
tinued study to be able to comprehend them aright. But the 
Naturalist can discern them with far less research in every plant 
that grows, in every animal that breathes ; and he meets with a 
constant variety which prevents him from growing weary of the 
pursuit. Yet the young are too frequently kept in ignorance of 
the wonders and beauties around them ; and, whilst encouraged 
to learn many languages, and read many books, they remain un- 
acquainted with the bright volume of Creation, the pages of which 
are daily and hourly unrolled before them, " written," to use the 
impressive words of Lord Bacon, " in the only language which 
hath gone forth to the ends of the world, unaffected by the con- 
fusion of Babel. But these pages are not to be read without 
some study : the alphabet and grammar must be learned, in order 
that their beauties may be rightly comprehended ; and those who 
are entering upon the inquiry need to be rightly directed by 
those who are more advanced. 

Natural History has been too generally shunned by those whose 
minds are occupied with the necessary employments and cares of 
the world, and who seek in the pursuit of knowledge a source of 
refreshment and relaxation, as a Science of hard names and intri- 
cate classification. But the objects of its several departments are 



INTRODUCTION. 1 1 

not commonly understood. The study includes the examination 
of the structure, habits, and mode of existence of all the living 
beings which so thickly people the surface of the globe; and it 
is only in order to become acquainted with these more readily, 
that the Naturalist arranges or classifies them, placing those to- 
gether which have most in common, and separating these from 
others which are widely different. Classification, therefore, is 
not the object of Natural History ; but a means of gaining that 
object ; and it is very easy to enter upon many interesting in- 
quiries without the slightest knowledge of it. The structure and 
actions of man, for example, may be examined in the greatest 
detail, without knowing any thing of his place in the general scale 
of being (although such knowledge will often shorten the student's 
labour ;) and other kinds of animals and plants may be observed 
in the same manner. In fact, several of the most valuable and 
interesting observations we possess upon the habits and actions 
of particular animals, were made by those who devoted them- 
selves almost exclusively to that special object. Thus it is 
scarcely out of the power of any one to contribute something to 
the general stock of knowledge ; still less, then, can any be pre- 
vented from adopting some department of this pursuit for the 
health and invigoration of their own minds. 

The study of the structure and actions of Plants, constituting 
what is known as Vegetable Physiology, has been less brought 
under the notice of those who pursue Natural History only for 
the improvement and recreation of their minds, than it perhaps 
deserves. In regard to* the importance of the Vegetable King- 
dom in the economy of Nature, it can scarcely be said to rank 
lower than the Animal Creation ; for all Animals are either directly 
or indirectly dependent upon Vegetables for their sustenance, 
and must cease to exist if they were destroyed. The beauty of 
the external forms of Plants is surpassed by that of their interna] 
structure ; and the investigation of the latter is more easy than 
that of animals, besides being unattended with many drawbacks 
which must elsewhere be encountered. The objects of the Phy- 
siologist are never out of reach ; for barren indeed must be that 



12 INTRODUCTION. 

country which affords no shelter to the products of the Vegetable 
Kingdom. The meanest and most common herbs are in his 
eyes as interesting as the majestic tree or the rarest flower. 
The toilsome labours of the Collector, who seeks to bring toge- 
ther in his cabinet as large a number as possible of the different 
tribes of plants existing on the surface of the globe, are not re- 
quired by him ; nor is his mind fatigued by the difficulties and 
technicalities of classification. And what renders the pursuit of 
this branch of Natural History peculiarly adapted to the female 
sex is its freedom from the necessity of that corporeal suffering, 
which, however laudable its ultimate objects, the truly humane 
will always dread to inflict upon beings that have feelings like 
their own. 

The object of the following Treatise will be, therefore, to lead 
those who may be disposed to adopt our recommendation, to a 
pursuit which cannot fail to prove a source of interest and im- 
provement. It will be adapted as much as possible to those who 
have no previous information on the subject, beyond that which 
all young persons of ordinary capacity may gain by themselves ; 
and it will omit, therefore, several topics of high but less general 
interest, which those who feel inclined to examine them will find 
fully treated elsewhere. 

Wherever circumstances are compatible with Vegetable exist- 
ence, there we find plants arise. It is not only on the luxuriant 
soil, on which many generations have flourished and decayed, 
that we find the display of their beauties. The coral island, but 
recently elevated above the level of the sea, speedily becomes 
clothed with verdure. From the materials of the most sterile 
rock, and even from the yet recent cinders and lava of the volcano, 
Nature prepares the way for vegetable existence. The slightest 
crevice or inequality is sufficient to arrest the invisible germs that 
are always floating in the air ; and the humble plants which spring 
from these soon overspread the surface, deriving their chief nutri- 
ment from the atmosphere. Having completed their allotted pe- 
riod of existence, they die and decay ; but their death is only a 



INTRODUCTION. 1 3 

preparation for the appearance of higher forms of vegetable struc- 
ture. They are followed by successive tribes of plants of gradu- 
ally increasing size and strength; until, in the course of years, the 
sterile rock is converted into a natural and luxuriant garden, of 
which the productions, rising from grasses to shrubs and trees, 
present all the varieties of the fertile meadow, the tangled thicket, 
and the widely spreading forest. 

No extremes of heat or cold seem to put an entire check upon 
vegetation. Even in the desert plains of the torrid zone, the eye 
of the traveller is often refreshed by the appearance of a few 
hardy plants, which find sufficient materials for their growth in 
these arid regions. And wherever a spring of water moistens the 
soil and atmosphere around, a spot of luxuriant verdure is found. 
These Oases, as they are termed, are the stations at which caravans 
halt, when crossing the extensive wastes of parching sand ; and 
although their effect upon the mind is doubtless heightened by the 
dreariness of the preceding journey, there is no question that few 
spots can present greater richness of vegetation than these. It 
will hereafter be seen that heat, light, and moisture combined 
form the circumstances most favourable to the growth of plants ; 
and it is from the combination of the latter of these conditions 
with the former, that the vegetation of small islands in the tro- 
pical ocean is so peculiarly rich. These Oases are like such 
islands in the midst of a sea of sand ; and nothing can be a 
greater contrast with the desolation around, than "the green 
pastures " and " still waters " which they afford. 

Many remarkable facts might be mentioned, relative to the 
degree of heat which some forms of vegetation are capable of sus- 
taining, and which, to some species indeed, appears a natural 
and even necessary condition. A hot spring in the Manilla islands, 
which raises the thermometer to 1S7° has plants flourishing in 
it and on its borders. In hot springs near a river of Louisi- 
ana, of the temperature of from 122° to 145°, have been seen 
growing not merely the lower and simpler plants, but shrubs and 
trees. In one of the Geysers of Iceland, which was hot enough 
to boil an egg in four minutes, a species of Char a has been fo imd 

2* 



14 INTRODUCTION. 

growing and reproducing itself; and vegetation of an humble 
kind has been observed in the similar boiling springs of Arabia 
and the Cape of Good Hope. One of the most remarkable facts 
on record, in reference to the power of vegetation to proceed 
under a high temperature, is related by Sir G. Staunton, in his 
account of Lord Macartney's embassy to China. At the island 
of Amsterdam a spring was found, the mud of which, far hotter 
than boiling water, gave birth to a species of Liverwort. A large 
Squill bulb, which it was wished to dry and preserve, has been 
known to push up its stalk and leaves, when buried in sand kept 
up to a temperature much exceeding that of boiling water. 

Even the extreme of cold is not fatal to every form of vegeta- 
ble life. In the realms of perpetual frost, the snow which covers 
mountains and valleys, and whose surface scarcely yields to the 
influence of the solar rays at midsummer, is in some places red- 
dened for miles together by a minute vegetable, which grows in 
its substance, and has been supposed, from its very rapid increase, 
to have fallen from the sky. This will be hereafter described 
under the name of Red Snow, which is that commonly applied 
to it. The Lichen which forms the winter food of the rein-deer, 
grows entirely buried beneath the snow ; and its quantity may 
be judged of by the number of the animals which find in it their 
sole support during a considerable part of the year. 

Plants are found, too, in situations in which some peculiar 
noxious influence might be supposed entirely to prevent their 
growth ; — as for example, in sulphureous springs. In fact, there 
are scarcely any circumstances in which there is not some kind 
of plant adapted to exist. Thus, it is well known that soils, whicn 
have any considerable admixture of metallic ores are not favour- 
able to most kinds of vegetation ; and among such, those mixed 
with the refuse of lead mines are the most sterile, so that this 
substance is often mixed with gravel, to prevent weeds from 
growing on garden-walks. Yet even on heaps of this material, 
thrown up around the openings of the mines, the Vernal Sand- 
wort thrives, growing perhaps even more luxuriantly than in any 
other situation. 



INTRODUCTION. 15 

The degree in which vitality is sometimes retained by plants, 
under the most unfavourable conditions, for a period to which 
it is difficult to assign a limit, is one of the most interesting and 
curious circumstances in their economy. In the greater part of 
those inhabiting temperate climates, an apparently complete ces- 
sation of activity takes place every year. The leaves wither and 
drop off; the stem and branches are reduced to a state of death- 
like barrenness ; and all the changes in which life consists appear 
to have entirely ceased. In some instances, the stems also die 
and decay, the roots only retaining their vitality ; yet from these, 
with the return of the genial warmth and light of spring, a new 
stem shoots up, and new leaves and flowers are produced, — in 
their turn to wither and decay. The torpor is not, however, so 
complete as it appears, in those durable and woody stems which 
defy the winter's blast ; for late experiments have shown that a 
slight movement of sap takes place even in a frosty atmosphere. 
In evergreen plants, on the other hand, this cessation of activity 
is less marked; but the difference between their summer and 
winter condition is much greater than is apparent. In all these 
cases, however, the changes are periodical ; and are not altogether 
dependent on external conditions. For nothing will prevent a 
plant from shedding its leaves nearly at its usual time; and al- 
though by artificial heat, or by removal to a warmer climate, a 
new crop can be brought out within a short interval, this exhausts 
its powers, so that few kinds can survive the change of circum- 
stances for any long period. Moreover, the period of inactivity 
cannot in these cases be prolonged beyond a certain fixed time ; 
for a plant whose growth in spring is checked by the protracted 
influence of cold, loses its vitality altogether. But there are some 
instances in which this condition may be greatly prolonged. Bulbs , 
for example, of the onion, hyacinth, tulip, &c. have been kept 
for many years in this dormant state, capable of renewing the 
active processes of vegetation, — of shooting op leaves and flower- 
stems into the air, and of transmitting their roots into the soil — 
for many years ; and there does not seem any particular limit to 
this power. Instances have been related of the growth of bulbs 



16 INTRODUCTION. 

unrolled from among the bandages of Egyptian mummies ; but 
there is reason to believe that deception has been practised on 
this point upon the too-ready credulity of travellers, — still, there 
is nothing impossible in the asserted fact. Light, warmth, and 
moisture are the causes of the growth of these curious structures ; 
and when removed from the influence of these, there is no reason 
why a bulb should not remain unchanged for 100 years if it can 
for 10 ; and for 1000 if for 100. We shall hereafter see that the 
vitality of seeds under similar circumstances appears quite un- 
limited. 

But there are some plants which, even whilst in a state of active 
vegetation, are capable of being reduced to a similar torpid con- 
dition, and of remaining in it for almost any length of time, 
without injury to life. There is a kind of Club-Moss inhabiting 
Peru, which is liable to be entirely dried up, when deprived of 
water for some time. It then folds in its leaves and contracts 
its roots, so as to form a ball, which, apparently quite devoid of 
animation, is driven about hither and thither by the wind; as 
soon, however, as it reaches a moist situation, it sends down its 
roots into the soil, and unfolds to the atmosphere its leaves, 
which, from a dingy brown, speedily change to the bright green 
of active vegetation. The Rose of Jericho is the subject of simi- 
lar transformations ; and the common Mosses exhibit the same 
in a less degree. 

These conditions are not only ones admitting of great varia- 
tion, and yet most important to the active operations of the vege- 
table structure. Light is as important as warmth and moisture 
to the processes of the economy; and yet we find plants adapted 
to thrive under the almost total deprivation of it. Sea-weeds 
possessing a bright green colour have been drawn up from the 
depth of more than 100 fathoms, to which the sun's rays do not 
penetrate in any appreciable proportion. Many of the Mush- 
room tribe have been found growing in caverns and mines to 
which no rays from the sun, either direct or reflected, would 
seem to have access ; and even more perfect plants have been 
observed to vegetate and to acquire a green colour (which is in 



INTRODUCTION. 17 

general only produced under the influence of strong light) in such 
situations. On the other hand, we find some plants adapted only 
to exist where they can be daily invigorated by the powerful 
rays of a tropical sun, with the complete daily change which re- 
sults from their total absence during a large part of the twenty- 
four hours ; whilst there are others whose energies, after remain- 
ing dormant during the tedious winter of the arctic regions, are 
aroused into a brief activity by the return of the luminary on 
whose cheering influence they depend, and whose rays are not 
withdrawn from them for weeks or even months together. Nei- 
ther of these tribes could flourish if transferred to the circum- 
stances of the other; and, opposite as these are, we observe that 
the Creator has adapted living beings to inhabit each, with equal 
suitableness. 

This adaptation of each species to particular circumstances is 
often seen in an interesting manner on a small scale, on the ex- 
terior of large trunks of trees, old towers, &c. which are thickly 
clothed with Mosses and Lichens. Many of these avoid the 
light ; and their presence indicates the north side of the body to 
which they are attaohed. To others, again, the light in all its 
strength is genial ; and they frequent the southern aspect ; whilst 
other forms, intermediate in habits, frequent the eastern and wes- 
tern sides ; so that, on going round such a tower or large trunk, 
we observe a succession of different species, which may be com- 
pared to that which is presented in the various latitudes, passing 
from the equator towards the pole. A similar succession on a 
larger scale is seen on ascending a high mountain between the 
tropics, such as the Peak of Teneriffe. The lower portion ex- 
hibits the vegetation of the surrounding country, in all the luxu- 
riance and richness of an island in the torrid zone. Higher up, 
the traveller meets with productions similar to those found on 
the borders of temperate regions ; and to these succeed those of 
the medium temperate zone. Above those are perceived the 
alpine plants, which in northern Europe are found at a compara- 
tively trifling elevation ; and to these succeeds the dreariness o! 
perpetual snow. These live distinct zones are well marked on 



IS 



INTRODUCTION. 



the Peak of TenerirTe ; each having a certain set of plants pecu- 
liar to it, as the plants of Northern and Southern Europe, and 
of Northern and Central Africa, are to those regions respec- 
tively. 

Thus we see that on no part of the earth's surface, under no 
peculiarities of soil or climate, is vegetation of some kind or other 
impossible. Every distinct tribe of plants flourishes naturally 
under peculiar conditions, — some preferring a warm atmosphere, 
others a cool one; — some only luxuriating in moisture, and others 
in the opposite condition of dryness; — some requiring the most in- 
tense light, and others only growing in darkness. There are some 
plants which are very deficient in the power of adapting them- 
selves to slight changes in these conditions ; and these are accord- 
ingly restricted to certain localities, which are favourable to their 
growth, and are hence considered rare plants. Thus, for ex- 
ample, there are certain species which require that the air sur- 
rounding them should contain a minute quantity of salt, dissolved 
in its moisture; — these only abound, therefore, near the sea- 
shore ; but they are seen to spring up in the neighbourhood of 
salt-works, even many hundred miles inland, — their seeds being 
conveyed by the wind or by birds, which have spread them over 
the whole surface of the earth, but there only meeting with the 
conditions they require for their development. On the other 
hand, there are many which can grow in almost any situation, 
and which can adapt themselves to a great variety of circum- 
stances, often exhibiting evident changes of form and aspect, 
which are due to the influence of these. Such are common 
plants ; and many of them are among those most serviceable to 
man, on account of the improvement which can be effected in 
them by cultivation. For example, the Potatoe, growing in its 
native climate — the tropical portion of South America, — does 
not require for the growth of its young shoots that store of 
nourishment which, in temperate climates, is provided in its 
fleshy tubers ; and the edible portion is thus extremely small, since 
the warmth and moisture constantly supplied to it develope the 
growing parts without such assistance. But when transplanted to 



INTRODUCTION. 1 9 

colder regions, and to a richer soil, that store is greatly increased 
in amount, and becomes one of the most important of all articles 
of food to man. If it were not for this capability of adapting 
itself to new circumstances, the plant could not thrive in North- 
ern Europe ; since its own powers of growth would be insufficient, 
when the external conditions are so much changed. But it is 
this very capability which renders it so useful to man. If the 
large Potatoes of European cultivation be planted again in tropi- 
cal climates, the produce is little superior to that of the original 
stock ; since, when circumstances no longer demand it, the ac- 
quired habit ceases. The Cabbage, Broccoli, Cauliflower, &c, 
are, in like manner, only varieties of one species, greatly altered 
by cultivation; the plant which was the original stock of all 
having been formed susceptible of more remarkable changes than 
most others, and thus rendered at the same time useful to man, 
and very easy of production. 

These instances, to which many more will be hereafter added, 
will suffice to show that it is not only in their original state that 
the adaptation of each tribe of plants to particular circumstances 
is exhibited ; since there are many which can thus spread them- 
selves, or may be spread by man, over a large part of the globe. 
And in this capability, no less than in their original aspect do 
we recognise the wisdom and power of the Almighty Designer, 
who willed that no portion of the globe should be unclothed by 
vegetation, and that from every part the herbage should spring 
forth for the supply of the Animal creation, which is entirely 
dependent on it, either directly or indirectly, for its sustenance. 

Such, then, being the universal diffusion of these beings, it is 
obvious that in no spot can he who seeks to make himself ac- 
quainted with their structure and habits, be without some subjects 
for examination. And since the humblest and simplest plants 
are found, when examined, to display an organization as remark- 
ably and beautifully adapted to the functions they are to perform, 
and to the conditions in which they are to exist, as is that of the 
highest and most complicated, there is no reason why any should 
be neglected, however insignificant they may appear. 



20 INTRODUCTION. 

The following volume is intended to serve as a guide to those 
who are inclined to make the wonders of the Vegetable kingdom 
an object of their regard, either as a source of recreation, or with 
those higher views to which the student of Natural History can 
scarcely avoid being led. For although no doubt can be enter- 
tained by the reflecting mind that the Power, Wisdom, and Good- 
ness of the Creator are every where operating with equal energy, 
whether in the simple but majestic arrangement of the heavenly 
bodies, or in those changes by which our own globe is rendered 
fit for the habitation of such innumerable multitudes of living 
beings, no one can help feeling that it is in the structure and 
actions of these beings themselves, that these attributes are more 
evidently manifested to the intelligent observer. And although 
the Animal kingdom has usually been regarded as affording 
more remarkable instances of their display than the Vegetable 
world, it may be doubted whether, when the latter is more 
closely examined, it will not appear equally or yet more won- 
derful. 



CHAPTER I. 

OF THE GENERAL CHARACTERS OF LIVING BEINGS, AND THE DISTINC- 
TION BETWEEN ANIMALS AND VEGETABLES. 

1. When we examine any common Vegetable, we find that 
it is composed of a number of parts, differing in their form and 
structure, — such, for example, as the stem, roots, leaves, and 

flowers. Each of these we might again subdivide into others; — 
the leaves, for example, into the footstalk on which they are sup- 
ported, and the expanded portion or blade. The blade of the 
leaf may be again distinguished into the midrib and the branch- 
ing veins proceeding from it (which form as it were its skeleton,) 
and the soft fleshy portion which clothes these ; and we might 
farther convince ourselves, by a little examination, of the presence 
of a kind of skin or cuticle, which envelopes the whole. Now 
these several parts of the structure of a plant, which have their 
respective uses in maintaining its life, — the roots, for example, 
being to suck up moisture from the soil through which they 
spread themselves, and to fix the whole structure in the ground, — 
the stem to convey this to the leaves, which it elevates into the 
air, and exposes to light and warmth, — the leaves to convert or 
elaborate this crude fluid into nutritious sap, — and the flowers to 
produce seed by which the being propagates its race, — these se- 
veral parts are termed the organs of which the plant is composed; 
and the uses of these parts — the changes they perform — are called 
their functions. 

2. Now it is in the presence of these different organs that one 
of the chief distinctions exists, between those structures which 
possess or have ever possessed life, and dead inert matter. In the 
stone or the mass of metal, we perceive that every part is similar to 

3 



22 GENERAL CHARACTERS OF LIVING BEINGS. 

every other part ; it has the same structure, the same properties. 
If it possesses the crystalline form, it may be reduced into an al- 
most indefinite number of smaller crystals similar to itself; and 
as to its properties, the chemist cares not (except as a matter of 
convenience) whether he examines a single grain or a mass of a 
ton weight. Nay, of many substances, the properties are so pe- 
culiar that they can be rocognised with certainty in quantities so 
minute as to be scarcely visible : thus, arsenic, when administered 
as a poison, has been detected after death in a quantity probably 
less than the hundredth of a grain ; and yet the experienced che- 
mist has no hesitation in asserting that this minute crystalline 
metallic substance is arsenic, because he recognises in it the 
same form and the same properties which a larger mass of that 
substance would exhibit 

3. Far different is it with regard to a Plant or Animal. These 
may be divided and subdivided ; but they then entirely lose their 
original character, for the parts or organs no longer bear any 
resemblance to the whole or to each other, either in form, struc- 
ture, or properties. Thus, then, we see that the bodies which 
are formed to exhibit those actions to which we give the general 
term of Life, are peculiarly distinguished from dead matter by 
the presence in them of a number of parts or organs, distinct 
alike in their form, structure, and properties; hence such are called 
organized bodies. On the other hand, dead inert matter may be 
divided with any degree of minuteness into parts similar to each 
other in form, structure, and properties ; hence it is termed inor- 
ganic, or destitute of organs. 

4. There is another peculiarity possessed by living beings in 
regard to their actions ox functions. Some of these actions are 
governed by the same laws as those which operate on inorganic 
matter ; the blood is propelled by the heart of an animal, for ex- 
ample, through its system of branching vessels, just upon the 
same principle that a forcing-pump drives water through the pipes 
which convey it over a large city. But the nature of the force 
is quite different. In the latter case it is merely mechanical. In 
the former it results from a property peculiar to organized struc- 



GENERAL CHARACTERS OF LIVING BEINGS. 23 

ture, and especially manifested in that form of it which is called 
muscle ;— the property, namely, of contracting, when a stimulus 
or irritation is applied to it. This and many other properties, 
therefore, which are exhibited by organized structures, and to 
which we see nothing analogous in inorganic matter, are termed 
vital; and it is by the operation of these properties, that the series 
of changes is produced, which constitutes the Life of any organ- 
ized being, whether Plant or Animal. Thus the heart has the 
property of contractility, which, when exercised, causes its con- 
traction ; — the eye has the property of receiving the impressions 
of light, which, when exercised, causes sensation; — and so on. 

5. It may be asked, — whence do these peculiar properties 
arise? Are living bodies composed of different elements from 
those which exist around us in the form of dead matter? Or are 
the elements the same, in a different state of combination 1 And 
can we attribute the peculiar properties of organized tissues to 
the peculiar state in which their particles exist? 

6. To this it may be replied, that there is no element entering 
into the composition of organized bodies, which is not also found 
in the world around ; and farther, that their chief elements are 
very few in number, compared with those which we find else- 
where. But the state of combination in which they exist is alto- 
gether peculiar, and such as the chemist cannot imitate, any more 
than the mechanic can imitate the arrangement of their parts. 
In fact, every organized structure with which we are acquainted, 
had its origin in another, which produced a germ capable of 
living and growing, and of constructing its peculiar fabric out of 
the materials it derives from the inorganic world ; and this again 
was produced by a former one ; — and so on. 

7. We perceive, therefore, that the living organized beings 
which we now witness around us, being all the descendants of 
others, whose succession we might trace backwards to their first 
parentage, their actions are as much the results of the general 
laws which the Creator of all impressed on the frame of His first* 
formed creatures, as are the movements of the planets round the 
sun, of the laws which lie impressed on them, when lie first set 



24 GENERAL CHARACTERS OF LIVING BEINGS. 

those glorious spheres in motion. These laws are continually 
maintained by His superintending agency, without which, all 
would be anarchy and confusion. 

8. It would seem to be a part of the exercise of those laws, 
that living beings should take from the inorganic world the mate- 
rials of their structure, — should convert these into parts of their 
own fabric, — should endow these with properties similar to those 
which their previous structures possessed,— and should even pro- 
duce from them the germs of new structures, capable of perform- 
ing the same changes. Thus, the germ contained in the seed 
builds up the beautiful form and wondrous structure of the perfect 
tree, with scarce any other materials than water and air ; and of 
these it not only constructs its own stem, leaves, roots, and flow- 
ers, but (what seems yet more extraordinary) it imparts to its 
seeds, which, when separated from it and dried up, seem as it were 
dead, the power of repeating for themselves the same operations. 
When once we understand it, however, as a general law, that it 
is a property of organized structures to produce the same, there 
is little difficulty in comprehending how they impart to the ele- 
ments they employ, properties so different from those which they 
previously possessed. For we find in every case, that a change 
of combination in these elements is attended with a change in 
their properties. Thus an acid (such as oil of vitriol) and an al- 
kali (such as soda) have properties peculiar to themselves, and 
in many respects contrary ; but when they are brought together, 
they unite into a new compound, which possesses a form and pro- 
perties differing from those of either of its elements. Again, sul- 
phur, nitre, and charcoal, when simply mixed together in certain 
proportions, form a product, gunpowder, which possesses proper- 
ties very different from those of either of its elements. Thus, 
then, we see that there is nothing improbable in the supposition, 
which all analogy supports, that the properties peculiar to organ- 
ized structures depend upon the peculiarity of their constitution ; 
and this peculiarity, which the chemist and the mechanic alike 
fail to imitate, results, as we have seen, from the general law, — 
that organized structures can only take their origin from beings 
already possessed of life. 



GENERAL CHARACTERS OF LIVING BEINGS. 25 

9. One more preliminary consideration must be adverted to, 
before we quit these general views. The properties of organized 
bodies require certain conditions for their operation. Thus, a 
seed, which possesses vital properties in a dormant or inactive 
condition, and which may retain these for hundreds or even thou- 
sands of years, if placed in favourable circumstances so to do, 
begins to germinate or grow, as soon as it is submitted to the 
proper- degree of warmth, moisture, and air. These, then, are 
the conditions requisite for those changes which we call its Life ; 
for the dry inactive seed can scarcely be said to be alive ; though, 
on the other hand, it certainly is not dead, since it possesses those 
properties or capabilities which enable it to live when placed in 
favourable circumstances. Again, suppose a plant to be actively 
vegetating under the influence of light, warmth, and moisture, and 
it be suddenly deprived of all these, — by being carried for exam- 
ple, into a cold dark cellar : — all its vital processes receive a check, 
and it either dies, or, if sufficiently hardy to sustain the shock, it 
remains inactive until the necessary conditions be renewed. 
These conditions are technically called the stimuli to vital actions ; 
and thus we see that Life is the result of the operation of these 
stimuli upon organized structures possessed of peculiar proper- 
ties. In attempting, therefore, to understand the history of Vege- 
tation, we have three things to consider ; in the first place, the 
nature of the structure of plants ; next, the properties which their 
several kinds of structure respectively possess ; and lastly, the 
operation of various external stimuli upon these properties, so 
as to produce vital actions. 

10. In considering the history of Animal Life, exactly the 
same course will be gone through ; but there will then be an ad- 
ditional subject to be treated of; namely the internal stimuli, 
arising from the will of the being, which cause those actions that 
are termed spontaneous, since they have no direct dependance 
upon external stimuli, but originate in the animal itself. In the 
history of Man, these actions evidently form a large part ; but in 
the lowest animals they are very obscure, and can often scarcely 
be distinguished from the actions of plants. But even in man we 



26 DISTINCTION BETWEEN PLANTS AND ANIMALS. 

have no difficulty in recognising a great number of actions ana- 
logous to those which constitute the whole life of plants. Thus, 
the absorption of food, its conversion into a nutritious fluid, the 
circulation of this through the system, its purification by exposure 
to the air, and the formation from it of new structures or the 
reparation of the old,— are all actions over which the mind and 
will have no direct control, which go on quite independently of 
it, and which may be regarded as perfectly analogous to the same 
functions in plants. Hence they receive the name of functions of 
vegetative or organic life ; whilst those of sensibility and power 
of spontaneous movement are termed functions of animal life, as 
being peculiar to that division of organized Nature. In fact it is 
by their presence or absence that the Animal or Vegetable cha- 
racter of a being must really be determined. For though the ex- 
ternal peculiarities of the higher kinds of Plants and Animals are 
quite sufficient to distinguish them from each other, yet there are 
many forms of the latter so low and simple, and so destitute of 
all that is regarded as peculiar to the Animal, that they cannot be 
readily distinguished from Plants. 

11. It is in these lowest forms of both kingdoms, that we re- 
cognise the nearest approach to inorganic matter. For we gra- 
dually lose, in descending the scale, nearly all appearance of dis- 
tinct organs ; so that the simplest plants — that, for example, which 
constitutes the Red Snow of Alpine and Arctic Regions (§ 48.) — 
instead of having stems, roots, leaves, and flowers, present us 
with apparently but a single organ, namely, a globular cell or 
little bag containing fluid. Even here, however, we shall subse- 
quently find that there is a distinction of parts ; and that, whilst 
the external surface is destined to imbibe nutriment from the 
moisture and air around, the internal forms the germs by which 
this simple little being is multiplied to a prodigious extent. 



CHAPTER II. 

GENERAL VIEW OF THE VEGETABLE KINGDOM. 

1 2. When we examine, however cursorily, the nature of the 
Plants around us, we at once perceive that their growth and suc- 
cession are regulated by certain laws. Thus we observe that all 
have a period of life to which they are more or less closely limited. 
Many of our commonest cultivated vegetables, — the Corn, the 
Beans, the Turnips of our fields, and many of the plants which 
enrich our gardens with their flowers, — live but for a single sum- 
mer ; springing up from seed, uprearing a lofty stem, putting forth 
expanded and luxuriant foliage, and unfolding gay and nume- 
rous blossoms, and finally withering away and undergoing com- 
plete decay, in the course of a few months. In others, on the con- 
trary, the duration of life is so great that it seems to be unlimited; 
but there is good reason to believe that the forest trees which lift 
their massive stems to the light of day through a succession of 
many hundred years, have an appointed limit to their lives as re- 
gular as that of man, — varying like his, in individual cases, ac- 
cording to the circumstances of each. Every plant, then, has a 
period allotted by the great Creator of all, for its springing from 
seed, the unfolding of its leaves, the expansion of its blossoms, 
and its subsequent death and decay ; but while death is the lot of 
each generation that " cometh up and is withered," the perpetua- 
tion of the race is accomplished by another law, which provides 
for the production by each individual, before its own dissolution, 
of the germs of new individuals, from which plants may arise, 
that go through their allotted period of life, and in their turn do- 
cay after producing the germs of a succeeding generation. 

13. Now besides these evident laws, another may be detected 
by a little observation, — that the beings produced from these 



28 



PERMANENCE OF VEGETABLE FORMS. 



germs are in every essential respect similar to their parents ; and 
that thus, after many thousands of generations, every plant or 
tree of the present day, may be regarded with certainty as having 
had a representative, at the period of the creation of the vege- 
tation which now clothes our globe. The exceptions which may 
seem to exist in regard to this law are so in appearance only. 
The seeds of any particular kind of Apple, for instance, will not 
produce the same kind with any certainty, but are as likely to 
give origin to trees that shall bear very different and far inferior 
fruit. The same may be said of the cultivated Dahlia, which 
presents so many beautiful varieties of colour ; the seed of a 
white flower is not much more likely to produce white Dahlias, 
than one with yellow or purple flowers. 

14. But in these and many more such instances, the different 
kinds are first produced by the influence of cultivation only, and 
had all originally but one stock ; and it is this stock, common to 
all kinds, which the seed has a tendency to perpetuate, rather 
than any one of the varieties which have been obtained from it 
by the art of man ; and we never find any tendency to produce 
a plant of an entirely different kind. Thus, the sour Crab is the 
stock of all the rich and delicate varieties of the Apple ; and if 
the seeds of any of these be sown in a poor soil, the plant will 
bear fruit resembling that of the original ; but still it will be an 
Apple, and never a Pear or a Quince, or any other of the kinds 
most nearly allied. In the same manner, the original stock of 
the Dahlia is a plant having a very ordinary yellow flower, with 
but one circle of coloured leaflets ; but by the influence of culti- 
vation the number of these circles is much increased, and the 
colours are deepened and enriched, as well as almost infinitely 
varied. The seeds of any of these, however, when sown in a 
poor soil, will produce a plant resembling the original parent; 
and thus it is seen that there is no real exception in such cases 
to the general law, — that the form of the species or distinct kind 
is propagated without any important alteration through succes- 
sive generations ; so that we may regard all the tribes of plants, 
really distinct from one another, as having existed in nearly the 
same form since their first creation. 



CHARACTERS OF SPECIES AND VARIETIES. 29 

15. The Naturalist, then, regards as distinct species those 
races of Plants, the differences between which are evident, and 
are such as are not likely to have resulted from cultivation or 
any other external cause, and do not exhibit any tendency to 
alteration in progress of years. Such, for example, are those be- 
tween the Apple and Pear among Plants, or the Dog and the Fox 
among animals. Among all the varieties of the Apple, different 
as they are from one another, there is none which exhibits any 
close resemblance to the Pear; and of all the kinds of Pear, there is 
none which so far loses its distinguishing characters as to show 
any great similarity to the Apple. And yet among the varieties 
of the latter, there are kinds more different from each other in 
size, shape, colour, flavour, &c. than some of these differ from 
the Pear ; but while all these show a marked tendency to change 
under different circumstances of growth, the internal differences 
between the Apple and Pear never exhibit any such tendency, 
but remain constant through all the varieties of each. The same 
may be said of the Dog and the Fox; for, though some varieties 
or breeds of the former seem to differ from each other more than 
from the Fox, yet these differences are liable to disappear alto- 
gether when the animals return to a wild state, all merging in a 
form most nearly resembling that of the shepherd's dog; whilst 
the differences between the Fox and the breeds of Dog most 
nearly allied to it are constantly manifested. 

16. On the other hand, the Naturalist regards as varieties of 
the same species Plants and Animals, in the various specimens of 
which, however dissimilar they may be, the points of difference 
exhibit such a tendency to variation, that the one kind passes, 
as it were, into the other. Thus, the Greyhound and the Bull- 
Dog would be regarded as springing from originally different 
stocks, if we did not meet with intermediate forms of the Dog 
which blend the peculiar characters of both. And the Primrose, 
Cowslip, and Polyanthus have been regarded as distinct species. 
so considerable are their differences in form and structure ; but 
the Botanist is now aware that many forms exist which are in- 
termediate between these, and that all may be raised from one 
stock. The same is the case with many other kinds of Plants. 



SO USE OF CLASSIFICATION. 

17. This explanation will, it is hoped, make the meaning 01 
the term species understood ; and it is very desirable that clear 
notions on the subject should be acquired by the student of Natu- 
ral History at the very commencement of his attention to the 
pursuit. It is computed that from 70,000 to 80,000 distinct spe- 
cies of Plants have been collected by Botanists from the surface 
of the globe ; and probably at least as many more remain to be 
discovered. It is obvious that an acquaintance with the struc- 
ture and characters of such a vast number of different races will 
be rendered much easier by classifying or arranging them,— 
placing those together which have a greater or less amount of 
general resemblance ; and separating others according to their 
amount of difference. It is only in this manner, indeed, that any 
one, within the compass of a single life, can become master of 
the whole. In making such an arrangement those species are 
first assembled into a group, termed a genus, which resemble 
each other in all the more important particulars, and differ only 
in minor details. For example, the different kinds of Roses 
among plants, and the Lion, Tiger, Leopard, and other species 
of the Cat kind among animals, are considered as belonging to 
the same genus, — their points of agreement being far more nu- 
merous than those of difference. Several genera may, in like 
manner, be united into a family ; the various members of which 
have a common resemblance, though with many subordinate dif- 
ferences. By continuing to pursue the same plan, we form divi- 
sions of greater and greater extent; until we are at last brought, 
by uniting subordinate ones, to the primary divisions into which 
the whole kingdom may be at once distributed, each of which 
exhibits a large number of very dissimilar groups, still united to- 
gether by some common points of general resemblance. 

18. Perhaps an illustration may make this subject better un- 
derstood. If we were to examine the people of any nation in 
which there had been but little intermixture among its different 
tribes, (as was formerly the case in Scotland in regard to the 
clans,) we might find a group of persons resembling each other 
so strongly in countenance, manners, form of speech, &c. and 
differing so much from all around them, that we should have lit- 



GENERAL DIVISION OF VEGETABLES. 31 

tie doubt that they belonged to one family ; and, going farther, 
we might meet with several such groups, each containing seve- 
ral individuals, and each differing in other characters from the 
rest. But if we were to bring these families together, we should 
probably be able to trace more general and less marked resem- 
blances among certain of these, which would lead us to associate 
them in clans, each of them including many families distinguished 
by certain points of similarity to one another, — as, for example, a 
strongly-marked feature or a peculiar dialect, — whilst differing in 
these same points from those of the remaining clans, and also 
differing from each other in minor points. Again, among these 
clans we might find some resembling each other and differing 
from the rest in their complexion or language, and thus forming 
tribes into which the whole nation might be subdivided. And, 
lastly, this nation would have certain points of conformity with 
those inhabiting the same quarter of the globe, whilst yet differ- 
ing still more strongly from them than its own tribes do amongst 
each other; and those inhabiting different quarters shall still 
more widely differ from each other, in general conformation, 
complexion, language, habits, &c. whilst still exhibiting those 
characters which are peculiar to man., and which separate him 
from all other animals. 

19. The primary division of the Vegetable Kingdom is into 
Phanerogamia or Flowering-plants, and Cryptogamia or Flower- 
less-plants. Though these designations are not strictly correct, 
they serve to indicate sufficiently well the character of the tribes 
to which they respectively apply. To the former division be- 
long nearly all cultivated vegetables, — the whole of the forest- 
trees both of our own and other countries, — and a very large 
proportion of the vegetation that naturally covers the surface of 
the earth in temperate and warm climates. Many of the tribes 
contained in it, however, produce no distinct blossom ; but these 
possess the essential parts of the flower (as will be hereafter ex- 
plained,) and form that perfect seed whieh is characteristic of 
this division. In all the Phanerogamia, (save in a few exceptions 
which stand, as it were, on the border of the division, and con- 



32 PHANEROGAMIA AND CRYPTOGAMIA. 

nect it with that of Cryptogamia, of which they exhibit some of 
the characters,) we find a certain number of distinct parts, — 
such as the stem, roots, leaves, and flowers ; and the germs by 
which they propagate their race come to an advanced state be- 
fore quitting the parent, and are furnished with a store of nou- 
rishment by which they are afterwards assisted in their growth. 
The seed of these plants is, therefore, a complex structure ; and 
the young plant shoots from it in a certain determinate manner. 

20. In the Cryptogamia, on the other hand, the parts con- 
cerned in the reproductive process are much less evident, and the 
germs which they form are much less matured when they quit 
the parent structure. In the Mosses, Ferns, Sea- Weeds, &c. no 
seeds are produced: but a number of small particles are liberated, 
which are termed spores; and each of these contains within it 
several minute germs which spring from it without any particu- 
lar regularity, and which are not assisted in their growth by any 
such store of nutriment as that provided in the seed. The ab- 
sence of this is a very important character ; for it seems a uni- 
versal law of Nature, that the higher the grade a living being is 
ultimately to attain, the longer is the period during which it is 
assisted, either directly or indirectly, by its parent, during the 
early stages of its growth. Thus Quadrupeds, which bring forth 
their young alive, and maintain them afterwards by suckling, are 
higher than Birds which produce them, in the first instance, in a 
state far less mature. And Man, who in his adult age rises far 
above all other animals, is longer dependent upon his parent 
during the period of infancy. 

21. The embryo of the flowering-plant, contained in the ma- 
ture seed, is so far advanced at the time of quitting its parent, 
that it possesses one or two distinct leafy bodies, termed cotyle- 
dons, which, when the seed begins to germinate (as it is called) 
are pushed up to the surface of the ground, and there turn to a 
green colour, and perform all the functions of true leaves, until 
these make their appearance. Now of all trace of these, the em- 
bryo of the flowerless-plant is entirely destitute ; and the whole 
group is hence spoken of as acotyledonous. On the other hand, 



CRYPTOGAMIA. FERNS. 33 

of the Flowering- plants some possess one and others two cotyle- 
dons; and this difference in the structure of the seed is accompa- 
nied by so many other differences in the structure of the stems, 
the leaves, flowers, &c. that it serves to mark the two principal 
subdivisions of this portion of the Vegetable Kingdom. That in 
which only one cotyledon exists is termed Monocotyledonous; 
and that in which there are two, Dicotyledonous. The common 
Bean or Pea will serve as a characteristic illustration of the 
former; and the Wheat and other Grass-seeds, of the other. 

22. The general aspect of the Flowering-plants is sufficiently 
well known to render a more minute account of them here unne- 
cessary ; since the object of this preliminary view of the Vegeta- 
ble Kingdom is to render the student, who may have been pre- 
viously entirely ignorant of the subject, prepared to enter with 
advantage on that detailed description of the mode in which the 
several tribes grow and reproduce themselves, which it is the ob- 
ject of the Physiological portion of this volume to communicate. 
A fuller sketch of the principal divisions of the Cryptogamia will, 
however, now be given, as few ordinary observers bestow much 
attention on them. 

23. Of all the Cryptogamia, the Ferns approach most nearly 
to Flowering Plants. The general aspect of those inhabiting 
this and other temperate countries is well known. They pre- 
sent a small number of leaves, — generally much divided into 
leaflets, and these again often minutely subdivided, — each arising 
from the ground by a woody stalk, which is commonly regarded 
as the stem of the plant. The true stem, however, is buried be- 
neath the ground, or sometimes creeps along its surface ; and the 
branches it sends upwards into the air are really the leaf-stalks. 
(Fig. 1.) In many Ferns of tropical Climates, the true stem rises 
upright, like that of a tree, and bears at the top a beautiful crown 
of those peculiarly graceful leaves for which the Ferns are re- 
markable. The height of these Tree Ferns, which are more luxu- 
riant in the small islands, where they are furnished with a more 
regular supply of atmospheric moisture than they can obtain at 
a greater distance from the sea, is sometimes as much as 40 or 

4 



34 



GENERAL CHARACTERS OF FERNS. 




Fig.1. 

Polypodium vulgare 

common Pol v pod y, 

or Wall-Fern. 



Fig. 2. Tree Fern. 



50 feet : so that we must not judge of the whole race by the com- 
paratively insignificant specimens which our own climate affords. 
These stems do not, however, afford any wood sufficiently solid 
to be employed in the arts. (Fig. 2.) 

24. The organs of reproduction in Ferns have no evident 
analogy with the flowering system in higher plants. Nothing 
like a flower is ever seen in this group ; and the fructification is 
incorporated, as it were, with the leaves, — being generally 
found, when mature, in brown spots or lines on their under sur- 
face or at their edges ; the nature of the organs composing these 
will be hereafter described. In most Ferns, all the leaves are 
concerned in reproducing the fructification; but in some (of 
which the Osmunda regalis, or Flowering-Fern as it is com- 



GENERAL CHARACTERS OF FERNS. 



monly but incorrectly termed, is an example) certain leaves are 
devoted to the production of the fructification, and are termed 
fertile; whilst others only perform the usual functions of leaves, 
and are called sterile leaves, from the absence of reproductive 
power in them. The term frond is generally applied to the 
leafy portions of the Cryptogamia, as distinguishing them from 
the true leaves of Flowering Plants, which have only one set of 
offices to perform. Sometimes the fertile frond of Ferns alto- 
gether loses its leafy aspect, its edges being completely rolled in 
so as to enclose the fructification ; and this separation of the re- 
productive from the nutritive portion of the system, is as com- 
plete as any which the Cryptogamia exhibit. 

25. One of the most interesting peculiarities of the Ferns is 
the spiral mode in which its leaflets and leaves are rolled up 
before their first appearance; each leaflet being rolled up to- 
wards the rib which supports it, — the ribs again towards the 
midrib, and the midrib towards, the footstalk. The unfolding 
leaves, in a state closely resembling those represented at the top 
of Figure 2, may constantly b© seen during spring in spots fre- 
quented by this group ; and, when examined, display the most 
provident and beautiful arrangement of the numerous minute 
parts of which the whole leaf consists. Few common objects, 
indeed, are more interesting than this, which requires neither 
skill nor the assistance of instruments for the detection of its 
beauties. 

26. Although Ferns constitute but a comparatively small 
part of the present vegetation of this country, they must have 
been much more abundant in a former period of the earth's his- 
tory, especially at the time when the beds of coal were being 
formed ; since their remains now constitute by far the largest 
part of those which are preserved to us with tolerable perfection 
in a fossil state. This is partly due, however, to the remarkable 
power which these plants possess, of resisting the action o\' 
water; by which other plants and trees were decomposed, — 
their remains having contributed to form those immense masses 
of Coal, which are so important to man, not only for his persona] 



36 



GENERAL CHARACTERS OF MOSSES. 



comfort, but for the arts of life. The Ferns are able to with- 
stand the effects of even a very prolonged immersion in water, 
with scarcely any change; whilst not only the soft tissue of 
plants, but the heartwood of most trees, decays so completely 
under the same circumstances as to leave little or no traces of 
their character. In tropical islands, the Ferns constitute a most 
important part of the whole vegetation ; being equal in number, 
in the Sandwich islands, to one-fourth, and in Jamaica to one- 
ninth, of all the Flowering plants existing in each of these locali- 
ties. 

27. The next principal group of Cryptogamia, that of Mosses, 
is as interesting from the delicacy and minuteness of ail the plants 
composing it, as other tribes of the Vegetable Kingdom are for 
the majesty of their forms, or the vast extension of their foliage. 
These are so generally and easily recognised as such, that a mi- 
nute description of them is at present unnecessary; but it should 
be stated that the term Moss is commonly applied not only to the 
true Mosses, but also to many Lichens. The true Mosses, how- 
ever, are always to be known by the green colour they possess 
except when dried up, white the Lichens are usually grayish in 

their aspect. Mosses usu- 
ally possess a sort of stem, 
round which the minute 
leaves are arranged with 
great beauty and regularity ; 
but neither this stem, nor the 
leaf-stalks of the leaves, have 
any truly woody structure ; 
and they more closely re- 
semble the simple tissue of 
j^ - the lowest plants, than the 
complex fabric of those al- 
ready noticed, to which they 
seem to bear a greater re- 
Mosses do not, like Ferns, bear 
their fructification upon the leaves or modifications of them ; it 




Fig. 3. Hypnum Castrensis, 
or Feather-Moss. 

semblance in external form. 



MOSSES. MUNGO PARK. 37 

is enclosed in a little case or urn, which is furnished with a lid, 
and is borne on a long distinct stalk, so as to be very easily ob- 
served when full-grown.. The interior of this minute organ usu- 
ally contains a structure of great beauty, which will be hereafter 
described in detail ; but it is interesting to know that it was by 
the contemplation of this that the heart of Mungo Park, the Afri- 
can traveller, was revived, when the difficulties by which he was 
surrounded had almost extinguished hope within him. The pas- 
sage has been often quoted ; but, it may be hoped, never without 
its use; and it does not seem superfluous to introduce it here. 

28. This enterprising traveller, during one of his journeys 
into the interior of Africa, was cruelly stripped and robbed of all 
that he possessed by banditti. " In this forlorn and almost help- 
less condition," he says, " when the robbers had left me, I sat for 
some time looking around me with amazement and terror. 
Which ever way I turned, nothing appeared but danger and 
difficulty. I found myself in the midst of a vast wilderness, in 
the depth of the rainy season, — naked and alone, — surrounded 
by savage animals, and by men still more savage. I was five 
hundred miles from any European settlement. All these circum- 
stances crowded at once upon my recollection ; and I confess that 
my spirits began to fail me. I considered my fate as certain, 
and that I had no alternative but to lie down and perish. The 
influence of religion, however, aided and supported me. I re- 
flected that no human prudence or foresight could possibly have 
averted my present sufferings. I was indeed a stranger in a 
strange land, yet I was still under the protecting eye of that Pro- 
vidence who has condescended to call himself the stranger's 
friend. At this moment, painful as my reflections were, the ex- 
traordinary beauty of a small Moss irresistibly caught my eye ; 
and though the whole plant was not larger than the top of one of 
my fingers, I could not contemplate the delicate conformation of 
its roots, leaves, and fruit, without admiration. Can that Being 
(thought I) who planted, watered, and brought to perfection, in 
this obscure part of the world, a thing which appears of so small 
4* 



38 



GENERAL CHARACTERS OF MOSSES. 



importance, look with unconcern upon the situation and suffer- 
ings of creatures formed after his own image? Surely not. — 
Reflections like these would not allow me to despair. I started 
up; and disregarding both hunger and fatigue, travelled for- 
wards, assured that relief was at hand, and I was not disap- 
pointed." 

29. Mosses are found in all parts of the world in which the 
atmosphere is moist ; but they are far more abundant in tempe- 
rate climates than in any between the tropics. They are among 
the first vegetables that clothe the soil with verdure in newly- 
formed countries ; and they are the last that disappear when the 
atmosphere ceases to be capable of nourishing vegetation. The 
first green crust upon the cinders with which the surface of 
Ascension Island was covered, consisted of minute Mosses. This 
tribe forms more than a fourth of the whole vegetation of Mel- 
ville Island, one of the most northerly spots in which any plants 
have been observed; and the black and lifeless soil of New South 
Shetland, one of the islands nearest to the South Pole, is covered 
with specks of Mosses struggling for existence. 

30. Besides their power of resisting extremes of temperature, 
Mosses exhibit a remarkable tenacity of life, when their growth 
is checked by the absence of moisture ; so that they may often 
be restored to active life, even when they have been dried for 
many years. Hence they offer abundant sources of interest to 
the observer of Nature, at a season when vegetation of other 
kinds is almost entirely checked. For it is most curious to ob- 
serve how gay these little Mosses are on every wall-top during the 
winter months, and in the early spring, — almost, or perhaps the 
only things which seem to enjoy the clouds and storms of the 
season. They choose the most exposed situations, spread out 
their leaves, and push up their delicate urns, amidst rain, frost, 
and snow, and yet there is nothing in their simple and tender 
structure from which we could infer their capability of resisting in- 
fluences so generally destructive to vegetation. But it is with 
plants as with Animals. — The more simple and lowly the being, the 



MOSSES. LIVERWORTS. 39 

greater is usually its tenacity of life under circumstances which 
depress the vital powers of higher kinds ; whilst the influences 
which they require are often too powerful for it. Thus, Mosses 
and Lichens, overstimulated by heat and dryness, wither away 
in summer ; but vegetate freely at a season when there is no 
other vegetation, and when their humble fabrics cannot be over- 
shadowed by a ranker growth. 

31. Mosses were fancifully termed by Linnaeus, servi, ser- 
vants or workmen ; for they seem to labour to produce vegeta- 
tion in newly-formed countries, where soil can scarcely yet be 
said to be. This is not their only use, however. They fill up 
and consolidate bogs, and form rich vegetable mould for the 
growth of larger plants, which they also protect from cold during 
the winter. They likewise clothe the sides of lofty hills and moun- 
tain-ranges ; and powerfully attract and condense the watery 
vapours floating in the atmosphere, and thus become the living 
fountains of many streams. They are sometimes, so completely 
dried up by drought, that they escape notice ; and then, when 
moistened by rain, they appear to have suddenly clothed a bar- 
ren heath or overspread a dry wall with verdure, on which, 
however, they really existed befora 

32. Closely connected with the Mosses is the tribe of Liver- 
worts, the lower forms of which are nearly connected with the 
Lichens. Some of them differ but little in their general charac- 
ters from Mosses, being distinguished by certain peculiarities of 
fructification. Others, however, have no distinct stem or sepa- 
rate leaves ; but extend horizontally into a flat leaf-like expan- 
sion ; the fructification is sometimes elevated above this on a little 
stalk ; but in the tribes most nearly allied to the Lichens, it is 
imbedded in it, as it is in that group. Their general habits 
closely resemble those of the Mosses. Their leafy expansions 
are soft and green ; differing much, therefore, from the dry scaly 
crusts of the Lichens. They are capable of reviving, like the 
Mosses, after being dried up ; and, from the rapidity of their 
growth, and a peculiarity in their mode of propagation, they are 
often seen to spread over a damp surface with great rapidity. 




40 LIVERWORTS. MARCHANTIA. 

One of the most common species is the 
Marchantia polymorpha, which will be 
often referred to in this treatise, on ac- 
count of the many interesting facts which 
the attentive study of it has disclosed. 
It is usually found growing on moist sur- 
faces, and often where there is little or no 
soil ; it is very common in the chinks be- 
tween paving-stones in unfrequented places 
and on the surface of the earth contained 
Marchantia polymorpha, ™ garden-pots as also upon walls which 
one of the commonest of from any cause are kept constantly damp, 
the Liverworts. 

33. Besides the regular fructification, this little plant has a 
very curious apparatus for the production of small leafy bodies, 
which may be regarded as buds, and which spontaneously sepa- 
rate from the parent structure and develope themselves into new 
beings. As these, when mature, are liable to be washed out of 
their receptacle by rain, and to' be carried to different parts of 
the neighbouring surface, and as they grow very rapidly whilst 
supplied with moisture, the rapid extension of the plant under 
such circumstances is easily accounted for. The little recepta- 
cles, of a basket form, in which these are produced, may be ge- 
nerally seen in some stage of their growth on the upper side of 
the leafy expansion of which the plant consists ; and they con- 
stitute beautiful objects for a low magnifying power of the micro- 
scope. The bud-like bodies, having the form of flat disks like 
coins, may often be seen to grow whilst still contained in their 
receptacle, and even to unite themselves, as it were, with the 
parent plant. 

34. The group of Cryptogamic plants termed Lichens are 
mostly dry, hard, scaly crusts, destitute of leaves and stems, and 
even of any thing bearing a resemblance to them ; they grow 
upon bare walls, the trunks of old trees, and other such situa- 
tions, in which they are much exposed to light, and not abun- 
dantly supplied with moisture. In their general structure they 




GENERAL CHARACTERS OF LICHENS. 41 

nearly approach to the Sea-weeds ; and differ from them chiefly 
in being adapted to live in air instead of in water. The dry hard 
crust is usually of a grayish colour ; its upper surface, being ex- 
posed to the light and warmth of the sun, performs the functions 

of leaves; whilst from beneath it 
there proceed a number of minute 
hair-Jike filaments, which serve both 
to fix it by clinging to the sub- 
stance on which it grows, and also, 
it may be believed, for the absorp- 
tion of fluid — the chief uses of the 
roots in the Flowering-Plants. 
Lichens are among the slowest in 

growth of all plants, and the least 

Pig. 5. & r .» 

Parmelia perforata, subject to alteration from decay. 

Lichen with projecting shields. Whilst alive, they scarcely exhibit 

any change through a long series of years ; and when dead, their 

forms and colours are scarcely altered by being dried. 

35. There can be no doubt that the greater part of this tribe 
derive their nourishment from the atmosphere and its contained 
moisture alone ; flourishing as they do upon sterile rocks^ with- 
out a particle of soil or mould in their neighbourhood. There 
are many species which ordinarily grow upon the trunks of 
trees ; and these are commonly spoken of as Mosses, — but incor- 
rectly so. The shaggy appearance of the apple-trees of an old 
Orchard is in general entirely due to Lichens, although a few 
Mosses may sometimes be found among these. Of such Lichens, 
by far the greater part vegetate indifferently on all kinds of 
trees, and they flourish equally well upon a damp wall ; so that 
there is no reason to suppose that they derive any more nutri- 
ment from the stems on which they grow, than is afforded by 
the moisture covering their surface. There is no doubt, how- 
ever, that some trees are much more favourable to their growth 
than others. Thus, the Beech, Elm, Sycamore, and Lime, are 
comparatively seldom found infested with the common Beard- 
moss, which clothes so profusely the Fir, Ash, Oak, or Birch ; 



42 MODE OF GROWTH OF LICHENS. 

so that the poet's epithet of "rude and moss-grown beach" is by 
no means appropriate. 

36. The fructification of the Lichens is not much raised above 
the general surface, but is usually imbedded in certain parts of 
it, somewhat differently formed from the rest, and termed shields. 
The early growth of these plants is favoured by darkness; but 
for the ripening of the reproductive bodies a considerable quan- 
tity of light is required. The development of the shields, which 
takes place under its influence, is frequently accompanied by so 
great a change in the general appearance of the plant, that the 
same species growing in dark and moist places, in which the 
fructification was not evolved, has been considered to belong to 
a distinct kind from the perfect specimen. No true Lichens are 
ever found in mines, caverns, or other places deprived of light ; 
nor are there any that grow entirely under water, although some 
species, which connect this group with the Sea- Weeds, grow on 
the sea-shore, where they are alternately submersed and left dry 
by the tide. 

37. To the Lichens may well be applied the title of Verna- 
culi or bond-slaves, which Linnaeus fancifully gave to the Sea- 
Weeds, regarding them as fettered to the rocks on which they 
grow. For the Lichens seem as it were chained to the soil which 
they labour to improve for the benefit of others, although they 
derive no nourishment from it themselves. The mode in which 
they prepare the sterile rock for the reception of plants which 
require a higher kind of nourishment, is most remarkable. They 
may be said to dig for themselves graves, for the reception of 
their remains, when death and decay would otherwise speedily 
dissipate them. For whilst living, these Lichens form a consi- 
derable quantity of oxalic acid, (which is a peculiar compound 
of carbon and oxygen, two ingredients supplied by the atmo- 
sphere ;) and this acts chemically upon the rock, (especially if of 
limestone,) forming a hollow which retains the particles of the 
structure, when their term of connected existence is expired. 
The moisture which is caught in these hollows finds its way into 
the cracks and crevices of the rocks ; and, when frozen, rends 



SOIL PRODUCED BY SUCCESSIVE TRIBES. 43 

them by its expansion into minute fragments, and thus adds more 
and more to the forming soil. Successive generations of these 
bond-slaves continuously and indefatigably perform their duties ; 
until at length, as the result of their accumulated toil, the barren 
and insulated rocks, or the pumice or lava of the volcano, become 
converted into fruitful fields. For when Flora's standard has 
once been planted on tracts thus claimed, they are soon colo- 
nized by plants of other tribes. The Mosses, Ferns, and other 
Cryptogamia follow them ; and, at last by the growth and decay 
of successive generations of plants, a sufficient thickness of soil 
is produced for the nourishment of the luxuriant herbage and 
the support of the lofty forest-tree. And thus, by the labours of 
these apparently insignificant plants, men are enabled to reap 
their harvest, and to supply themselves with timber from the 
forests, and cattle increase and multiply, on what was formerly 
but a naked and desolate rock. 

38. One of nature's truest though least attractive delineators 
has thus faithfully described such a process as it occurs on ruined 
buildings. It should be remarked, however, that the terms seed, 
foliage, and flower, are not strictly correct as applied to the 
Lichens, which have none of these. 

"Seeds to our eyes invisible, will find 
On the rude rock the bed that fits their kind; 
There in the rugged soil they safely dwell, 
Till showers and snows the subtle atoms swell, 
And spread th' enduring foliage; then we trace 
The freckled flower upon the flinty base ; 
These all increase, till in unnoted years 
The stony tower as gray with age appears, 
With coats of vegetation thinly spread, 
Coat above coat, the living on the dead. 
These then dissolve to dust, and make a way 
For bolder foliage, nursed by their decay : 
The long-enduring ferns in time will all 
Die and depose their dust upon the wull : 
Where the wing'd seed may rest, till many a flower 
Shows Flora's triumph o'er the falling tower." 

Crabbk's DoRonTir. 



44 



USES OF LICHENS. ALGJE. 



39. Besides this important office in the economy of Nature 
some of the Lichens are peculiarly useful to man, on account of 
the valuable dyes they afford him. The blue dye termed Archil 
or Litmus, which is changed to a bright red by the action of acids, 
is obtained from a species of Lichen growing in the Canary 
islands ; and many other species not at present regarded might 
probably be converted with advantage to the same use. To the 
Laplanders the tribe of Lichens is of peculiar utility ; indeed on 
it they depend for their subsistence. For though it is not an ar- 
ticle of their own diet, a humble Lichen commonly known as the 
Reindeer Moss supplies the animal, on which they depend for 
almost all their means of existence, with food throughout their 
dreary winter, its vegetation not being checked by the snow be- 
neath which it grows. A species of Lichen growing on the rocks 
of the Arctic regions of North America, has afforded subsistence 
for many days to some of the adventurous explorers of that de- 
solate country, when other provisions could not be obtained. 

40. The group of JHgae, or Sea- Weeds, includes the very 
lowest forms of vegetable organization ; but it also comprehends 
some plants whose structure possesses great complexity. The 
Algae may be considered as Lichens formed to exist in water ; 
their general structure, and the arrangement of their parts being 
much alike. The hard scaly crust of the Lichens, formed under 
the influence of the sun and air, and never attaining any great 
extent, seems to bear a remarkable contrast with the immense 
leaf-like expansions, composed of soft, easily decomposed tissue, 
presented by the Algae ; yet wherever any of the former group 
inhabit damp shady places, their character much approaches that 
of the latter ; and in regard to some plants, it is difficult to fix the 
group to which they belong. Although the term Sea-Weed is 
that usually considered equivalent to Algae, it should be under- 
stood that the class includes many species which are inhabitants 
of fresh water. Of this kind are the Conferva?,— the long green 
hair-like filaments of which are almost constantly found attached 
to stones at the sides or bottom of running streams. These are 
among the simplest forms of vegetation. Each filament consists 



ALGiE, OR SEA-WEEDS. 



45 




Fig. 6. Confervae, 



of a single row of minute cells or vesicles, attached to each other 
end to end. Each of these vesicles is capable 
of growing by itself, and of reproducing its 
kind ; for at a certain period a minute orifice 
appears in its walls, from which issue forth 
some of the little green particles it contains ; 
and these become the germs of new plants of 
the same description, 

41. The higher kinds of Algae inhabit Sea- 
water only. They often assume the forms of 
more perfect plants, presenting an appearance 
as of roots, stems, and leaves. But these parts 
have not those differences of structure which 
are characteristic of them when truly formed, 
and which will be hereafter described ; on the 
contrary, they all consist of the same kind of 
with separate fila- simple and similar texture as that of the Con- 
ment magnified . fervdd ; — the expanded leaf of a Sea- weed being 

composed, as it were, of a num- 
ber of filaments of the Conferva? 
laid side by side. The struc- 
ture of these apparently different 
parts being thus so nearly he 
same, their functions or uses 
have an equal conformity; for 
"^^cgL the root-like fibres at the bot- 
tom of the stem only serve to 
fix the plant to the rocks or 
stones to which it is its habit to 

attach itself, instead of absorbing 
Fi<r. 7. Alaria csculcnUi, , . . 

a species of Sea-wecd having apP a. or sucking up nourishment as 

rcntly a stem with roots and leaves ; j n the flowering plants. The 
thesnmllngureisanK.g.niiedviewof rf ^ difference is ob- 

a section of a small part, showing the 
interior structure. vious. Where the ir/iolc plant 

is constantly immersed in the fluid which affords it the materials 

of its growth, no one part of it need be specially endowed with 

5 




46 MODE OF GROWTH OF SEA-WEEDS. 

the power : and it will be hereafter shown (Chap. IV.) how strong 
the contrast is between the functions of the true roots of Flower- 
ing plants and the root-like organs of the Algae. 

42. The higher Algae sometimes attain a prodigious extent 
of development, forming vast sub-marine forests of the most luxu- 
riant vegetation. Thus the Chorda Filum, a species common 
in the North Sea, is frequently found of the length of 30 or 40 
feet ; and in the neighbourhood of the Orkneys it forms meadows 
through which a boat forces its way with difficulty. It grows in 
the form of a long and even cord (whence its name) about the 
size of a quill, attached at one end to the bottom or shore, and 
the rest supported by the water. This is nothing, however, to 
the prodigious extent of the Macrocystis pyrifera, which is re- 
ported to be from 500 to 1500 feet in length, the long and nar- 
row fronds having an air- vesicle at the base of each, the stem not 
being thicker than the finger, and the upper branches from it as 
slender as common pack-thread. Another tropical species attains 
the length of 25 or 30 feet, with a trunk as thick as a man's 
thigh. Sometimes these stems are solid, and sometimes hollow ; 
the tubular stem of one species of Laminaria, found near the Cape 
of Good Hope, has been used by the natives as a trumpet when 
dried. Another species furnishes the natives of some parts of 
Australia, with a large proportion of their instruments, vessels, 
and even of their food. 

43. The marine Algae differ much in their habits. Some 
species grow altogether beneath the water, attaching themselves 
below the lowest tide-level. Others fix themselves where their 
fronds may float on the surface, and be exposed in some degree 
to the direct influence of the air. Others again frequent a height 
at which they are left dry at every retreating tide ; and some 
are found in situations in which they are scarcely ever covered 
by water, thus approaching in habits, and in character also, to 
the Lichens. Although most attach themselves to rocks or other 
solid masses, frequenting the shores or shallows rather than the 
open sea, there are some exceptions, among which one of the 
most remarkable is the Sargasso or Gulph Weed, which floats 
on the surface of the ocean, in the Gulph of Mexico, and in the 



HABITS OF SEA-WEEDS. 47 

current which sets from it towards the north. Immense fields of 
it are seen by the navigator, extending as far as the eye can reach, 
and conveying the idea of rocks and shallows, — dangers far dis- 
tant. It is sometimes so abundant as seriously to interfere with 
the progress of the ship through the water ; and it was this which 
alarmed the crew of Columbus, in his first voyage of discovery. 

44. The distribution of different species through the ocean is 
influenced by latitude, by the depth of water, and by currents, 
nearly in the same manner as the higher plants are affected by 
temperature, elevation above the sea-level, and the conditions of 
the atmosphere as to dryness and calmness. Some species can 
thrive well under considerable variation in these conditions ; 
whilst others are dependant upon certain states of them for their 
existence. The former, therefore, are extensively diffused, being 
found along many shores, whilst the latter are rarer, and only 
inhabit particular spots, in which these conditions are met with. 
Contrary to what might have been expected, — considering that 
the Algae do not imbibe any nourishment by the spreading root- 
like fibres which attach them to the solid masses of the shore, — 
it has been ascertained that they do not grow indifferently on all 
kinds of rocks ; but that if, for example, along the same line of 
coast, there be an alternation of limestone and granite rocks, 
some species will attach themselves in preference to the former, 
and others to the latter. This curious fact can only be explained 
by the supposition that small quantities of the mineral matter are 
dissolved by the water of the neighbourhood ; and that in this 
manner they act upon the plant. 

45. Of all tribes of plants, the Algae are commonly reputed 
the least useful ; in fact their inutility was proverbial among the 
ancients. Yet neither in regard to the general economy of na- 
ture, nor as to the wants of man, are they to be so considered. 
They supply food to a large number of marine animals, which 
browse upon them as those inhabiting the land do upon its most 
luxuriant pastures. Cattle have been very profitably fed on 
some species abundant on the northern shores ; nnd'even become 
so fond of this diet as greedily to seek for it. Many kinds furnish 
a wholesome and palatable food for man, and arc employed by 



48 



USES OF SEA-WEEDS. 



the poorer classes along the shores of the North of Europe ; 
whilst others are reckoned a luxury by the rich. The Laver of 
this country, the Carrageen, or Irish Bog Moss (as it is erro- 
neously called,) and other edible substances, belong to this 
group ; and from other species of it are formed the edible birds'- 
nests which are considered so great a delicacy by the Chinese, — - 
the best being sold for nearly their weight in gold. These nests 
are constructed by a bird resembling the Swallow, which re- 
duces the Sea- Weed in its stomach to a sort of gelatinous mass, 
before employing it for this purpose. 

46. But all these uses are comparatively trifling, when the 
other modes in which the Alg33 may be made beneficial to man 
are considered. The kelp, from which until recently the glass- 
maker and soap-boiler derived most of the alkali which they re- 
quired for their manufacture, is nothing but the ashes of Sea-weeds, 
which contain a large proportion of this substance, derived from 
the water in which they grow. Other means of obtaining soda 
from sea-water have now partly superseded this ; but until re- 
cently it was almost the only method. The account handed 
down by tradition of the mode in which glass was invented, 
whether it be itself true or false, serves to illustrate the properties 
of the Sea- Weed. It is said that some sailors cast ashore by 
shipwreck, having kindled a fire on the sand, supplied it with 
some dry sea- weed as fuel ; and that under the ashes a mass of 
vitrified matter was afterwards found, resulting from the union 
at a high temperature, of the soda of the sea- weed, with the silex 
of the sand. Many Algae also constitute a very valuable manure ; 
and might be much more used than they are. But one of their 
greatest benefits to man consists in the Iodine with which they 
supply him ; — a substance which is of the most important use to 
the physician in the treatment of many diseases, and which is a 
nearly certain cure for some which were formerly considered 
almost irremediable. One species, moreover, which abounds on 
the shores of China, furnishes a glue and varnish to the Chinese* 
even superior to that which is obtained from animal matter in 
this country. It seems, when once dried, to resist the action of 
water ; for it is employed to fill up the lozenge-shaped interstices 



PECULIAR TRIBES OF ALGJE. RED SNOW". 49 

in the network of Bamboo of which windows are frequently 
constructed ; as well as to strengthen and varnish the paper of 
their lanterns. A species abounding on the southern and west- 
ern coasts of Ireland furnishes a good size for house-painters ; 
and there are many others which contain an amount of gelati- 
nous matter that might be rendered useful in various ways. 

47. Besides the tribes of whose character a sketch has been 
thus given, there are others of a doubtful nature, which are gene- 
rally referred to this group ; although some peculiar characters 
which they exhibit, and their similarity to certain animal forms, 
render it doubtful whether they ought not to rank with that king- 
dom. They are mostly formed of cells jointed together, as the 
Conferva} ; but some of them seem to possess a different interior 
structure ; and others exhibit very curious motions, which can 
scarcely be distinguished with certainty from those of animals. 
In one of these groups, a large quantity of flinty matter is con-, 
tained in the walls of the cells ; so that they perfectly retain their 
form after all the vegetable structure has been destroyed by the 
action of heat and acidSi The cavity of the cells, too, is some- 
times seen to be partly occupied by large angular crystals. All 
the plants (if such they be) of this group are very minute. 

48. There is, however, a group yet simpler than these, of the 
vegetable nature of which there is no doubt. On the damp parts 
of some hard surfaces is not unfrequently seen a greenish or red- 
dish slime, which, when examined with the microscope, is found 
to consist of a number of minute cells, having little connexion 
with each other, but imbedded in a sort of jelly, which surrounds 
and connects them. On some minute variations between these 
simple plants, various distinctions have been formed; one is 
known under the name of gory dew, from its red colour ; and 
another, which appears on the surface of snow, tinging extensive 
tracts with a deep crimson, is known as red snow. This some- 
times appears so suddenly, and over so large a space, as to lead 
to the belief that it had fallen from the sky ; but its growth and 
multiplication are so rapid as to leave no difficulty in accounting 
for its appearance. This plant, which may be regarded as one- 




50 RED SNOW. FUNGI. 

of the simplest forms of vegetation, if not the very simplest, con- 
sists of a little bag or membrane, forming 
what is called a cell. A large number of 
these are commonly found together ; but each 
one is separate from the rest, and is to be re- 
Fig. 8, garded as a distinct individual. It obtains its 

Protoccocus nivalis, own nourishment by absorbing the fluid 
or Red Snow, highly , _ - 

mao-nified; showing around, and grows and comes to maturity 

its separate cells or without any other support or assistance than 
bedded 3 in* a "slimy ^at afforded by the air and moisture with 
jelly, which its surface is in contact. When come 

to maturity, a number of minute granules may be seen within 
it; these are the germs of new plants ; and, when liberated by 
the rupture of the parent-cell, they go through precisely the same 
series of changes. This little plant will be often referred to in 
illustration of the simplest conditions in which the processes of 
the vegetable economy can be performed. In its habits, — flou- 
rishing as it does only in very damp situations, though partly ex- 
posed to the air, — it must be regarded as belonging to the Algae ; 
but it bears a close correspondence with the lowest forms of a 
group that now remains to be considered, whose conditions 
of existence, however, are very different. 

49. In their genera] simplicity of structure, the Fungi, (the 
tribe including Mushrooms, Puff-balls, and many 
kinds of blight, mildew, and mould,) correspond 
with the Algse and Lichens; but they differ remark- 
ably in habits, and in the character of their fabrics. 
Fungi will not grow with the simple nourishment 
which serves for their support, but require to be 
fed with decaying animal or vegetable matter of 
some kind ; and they chiefly frequent situations in 

■ ' . which decomposition is going on with rapidity, and 
Common Blue r ° ° 

Mould greatly which are at the same time dark and warm. It is 

magnified; its ver y remarkable to observe the constancy with 

stems consist- , -, 

ing of single which particular species make their appearance on 

cells loosely par ti C ular substances. Thus, no fungus but the 
iointed toge- 
ther common edible Mushroom ever grows upon the 




PROPAGATION OF FUNGI. 51 

mushroom-spawn (as it is called;) though this does not contain 
its germs, being merely a kind of manure composed of various 
decaying substances which prepares the soil to receive them from 
the atmosphere. Again, there is a species of mould which is only 
found on the surface of the dung of cats deposited in moist and 
obscure places. Almost every tribe of plants has its peculiar spe- 
cies of blight or rust, to the attacks of which it is liable, and which 
differ from the kinds infesting nearly similar vegetables. 

50. The universality of the appearance of the simpler kinds of 
Fungi, — such as mould, mildew, &c. — upon all spots favourable 
to their development, has given rise to the belief that they were 
spontaneously produced by the decomposing substances. But 
there is no occasion for this mode of accounting for it; since the 
extraordinary means adopted by Nature for the production and 
diffusion of their germs suffices to explain it. The duration of 
the lives of individuals among the Fungi is very brief; the tissue 
is soft and succulent, sometimes containing so little solid matter 
as almost to melt away when broken down ; and never posses- 
sing any considerable amount of firmness. Now in the Algae, 
where we have seen the development of the individual taking 
place to such an enormous extent, the fructification is generally 
obscure, and sometimes even scarcely perceptible. But in the 
Fungi, all the energies of the plant seem directed to the produc- 
tion of the germs of new ones ; its own size seldom attains any 
great extent ; but the number of these germs is often almost in- 
calculable. Thus, the fine dust which issues from the common 
Puff-ball when mature, consists entirely of these little bodies, 
which are diffused through the air, and seem to float about in it, 
ready to develope themselves when they meet with the fitting 
conditions. In a single Fungus above ten millions have been 
counted ; and these were probably by no means the whole num- 
ber contained in it. When these minute germs are once spread 
through the air, there are so many means provided lor their dif- 
fusion, that it is difficult to conceive of a place from which they 
should be excluded. 

51. However improbable, then, it may at first sight appear, 



52 DEVELOPMENT OF MOULD, MILDEW, &C. 

that every portion of the air we breathe should contain the germs 
of a large number of species of Fungi, ready to develope them- 
selves whenever the peculiar conditions adapted to each kind are 
presented, there seems good reason to believe that such is the 
case ; and in this manner we may account for several facts of 
some practical importance, relative to the production of those 
very troublesome forms of vegetation known by the names of 
mould, mildew, &c. It is well known that fruit-preserves are 
very liable to be attacked by the common bead-mould ; which 
no care employed in completely closing the mouths of the jars 
can prevent. It has been remarked, however, that they are much 
less liable to suffer in this way, if not left open for a night before 
they are tied down ; and it is therefore probable that the germs 
of the mould sow themselves, as it were, in this luxuriant soil, 
before the jar is covered. Again, there is a particular kind of 
cheese, much valued by some epicures, which derives its peculiar 
flavour from the quantity of fungous vegetation it contains. It 
is prepared simply by breaking up the curd, and exposing it for 
a day or two, in small lumps laid upon a cloth, to the sun and 
air ; it there seems to receive the germs of Fungi, which after- 
wards vegetate in it, and spread their growth through the mass 
whilst it is yet soft. 

52. In all these instances, the Fungi derive their nutriment 
from organic matter which is either already in a state of decay, 
or will readily decompose. There can be little doubt that their 
development hastens decay when it is slow, or even causes de- 
composition in substances which previously exhibited none. 
Thus, a fruit-preserve, in which no mould finds its way, may 
remain sweet for many years ; but the growth of the mould pro- 
duces chemical changes in it, which are of a kind to supply the 
plant with the materials it requires. There is another very re- 
markable group of Fungi, which developes itself in the midst of 
the tissues of living plants and animals. To it belong, amongst 
others, the mildew, rust, smut, &c. of corn, which are distinct 
plants, having all the characters of true Fungi, but growing from 
the ears, stems, &c. of those they infest, so as to appear like a 



FUNGOUS VEGETATION IN ANIMALS. 53 

part of themselves. In fact the question has been raised whether 
they are really produced from separate germs, or whether they 
are not diseased parts of the structure on which they appear. 
But there seems little doubt that distinct germs are introduced 
from without. They can be communicated from one plant to 
another ; and they may perhaps enter through the stomata or 
breathing pores hereafter to be described; though experiment 
shows it to be more likely that they are conveyed in the water 
which drains through the soil, and that they are introduced into the 
system with the fluid which is absorbed. In that case they must 
be almost immeasurably small ; since it is known that the mi- 
nutest particles of any substance which can be artificially ob- 
tained, are usually rejected by the roots, as too large, when dif- 
fused through water which is being absorbed through their 
pores. 

53. Animals are liable, as well as plants, to the growth of 
Fungi within their bodies. There is a species of Wasp in the 
West Indies, of which individuals are often seen flying about with 
plants of their own length projecting from some part of their sur- 
face, the germs of these having been originally introduced, pro- 
bably through the breathing pores at their sides, (which greatly 
resemble those of plants,) and taking root, as it were, in their 
substance, so as to develope a luxuriant vegetation. In time, 
however, the fungous growth spreads through the body, and de- 
stroys the life of the insect ; and then it seems to grow more 
rapidly, the decomposing tissue of the dead body being still more 
adapted than the living structure to afford it nutriment. 

54. A very curious example of the growth of Fungi within 
the living animal body has lately been detected ; and the know- 
ledge of it has proved of great importance. The Silk-worm 
breeders of Italy and the South of France, especially in particular 
districts, have been subject to a considerable loss by a disease 
termed Muscardinc, which sometimes attacks the worms in large 
numbers, just when about to enter the chrysalis state. This dis- 
ease has been ascertained to be due to the growth of a minute 
vegetable of the Fungus tribe, nearly resembling the common 



54 MUSCARDINE OF SILK-WORMS, — YEAST. 

mould, within their bodies. It is capable of being communicated 
to any individual from one already affected, by the introduction 
beneath the skin of the former of some particles of the diseased 
portion of the latter ; and it then spreads in the fatty mass be- 
neath the skin, occasioning the destruction of this tissue, which is 
very important as a reservoir of nourishment to the animal when 
about to pass into a state of complete inactivity. The plant 
spreads by the extension of its own structure ; and also by the 
production of minute germs, which are taken up by the circu- 
lating blood, and carried to distant parts of the body. The dis- 
ease invariably occasions the death of the Silk- worm; but it does 
not show itself externally until afterwards, when it rapidly shoots 
forth from beneath the skin. The Caterpillar, Chrysalis, and Moth 
are all susceptible of having the disease communicated to them 
by the kind of inoculation just described ; but it is only the first 
which usually receives it spontaneously. The importance of this 
disease to the breeders of silk-worms, led, as soon as its true 
nature was understood, to careful inquiry into the circumstances 
which favour the production of the fungus; and it has been 
shown that, if the bodies of the caterpillers, which (from various 
causes) have died during breeding, be thrown together in heaps, 
and exposed to the influence of a warm and moist atmosphere 
for a few days (as has been very commonly the case,) this fungus 
almost invariably appears upon them, just as other kinds of mould 
appear on other decaying substances ; and that it is then propa- 
gated to the living worms by the diffusion of its germs through 
the atmosphere. The knowledge of this fact, and the precau- 
tions taken in consequence, have greatly diminished the mor- 
tality. 

55. Another very curious example of vegetation of a fungous 
character in a situation where its existence was not until re- 
cently suspected, is presented in the process of fermentation. It 
appears from microscopic examination of a mass of yeast, that it 
consists of a number of minute disconnected vesicles, which 
closely resemble those of the Red Snow, and appear to consti- 
tute one of the simplest possible forms of vegetation. These, like 



VEGETATION OF YEAST. 55 

seeds, may remain for almost any length of time in an inactive 
condition without losing their vitality; and their power of grow- 
ing when placed in proper circumstances is not destroyed by 
being entirely dried up, nor by being exposed to such extremes 
of temperature as the boiling point of water and seventy-six de- 
grees below zero. When these bodies are placed in a fluid 
in which any kind of sugary matter is contained, they commence 
vegetating actively, provided the temperature be sufficiently high ; 
and the decomposition which they effect in the fluid, the nature 
of which will be presently explained, is that which constitutes its 
fermentation. 

56. If a small portion of a fluid in this state be examined at 
intervals with a powerful microscope, it is observed that each of 
the little vesicles contained in it puts forth one or more pro- 
longations or buds, which in time become new vesicles like their 
parents ; these again perform the same process ; so that, within a 



© 



%N°, 



© 



C b, 




O 
O o Co 

a b 

Fig-. 10. Different stages of the vegetation of Yeast; a, single cells 
of which it at first consists; 6, cells with buds; c, the same more 
advanced; d, rows of cells corresponding to those of Fig. 9. 

few hours, the single vesicles have developed themselves into 
rows of four, five, or six. This is not the only way, however, 
in which they multiply ; for sometimes the vesicles are observed 
to burst, and to emit a number of minute granules, which are the 
germs of new plants, and which soon develope themselves into 
additional cells. By the time that five or six vesicles are found 
in each group, the fermentation is sufficiently for advanced for 
the purposes of the brewer; and he then takes measures to check 
it, by which the vegetation of the yeast is suspended. The 
groups of vesicles then separate into individuals resembling those 
which first constituted the yeast ; and thus a greatly-increased 
amount of this substance is the result of the process. 



56 IMPORTANCE OF FUNGI. 

57. The process of fermentation consists, as regards the fluid, 
in the conversion of the solution of sugar into alcohol or spirit of 
wine ; and this is effected by the action of the vegetating fungus, 
which withdraws from the fluid, for the supply of its own growth, 
that portion of its elements which constitutes the difference be- 
tween sugar and alcohol. A process very similar to this takes 
place when the common Mould, growing upon the surface of a 
sweet preserve, causes its fermentation. The little plant bears 
considerable resemblance to the Red Snow ; but differs from it 
in the following two important particulars. 

58. The Red Snow can flourish when supplied with air and 
moisture alone, — the conditions which have been mentioned as 
favourable to the growth of the Algae ; whilst this Yeast-plant can 
only grow in the solution of vegetable matter which is ready to 
undergo decomposition, and to yield it a kind of nutriment which 
the Red Snow does not require, but which is necessary for the 
growth of all the Fungi. This is an instance, then, of what was 
formerly stated respecting the close resemblance between the 
lowest forms of these simple tribes, which differ from one another 
more in the conditions necessary for their respective growth, 
than in their own structure. The other point of difference con- 
sists in the extension of the Yeast-plant by buds, that is, by the 
formation of new cells as continuations from the old one, as well 
as by the formation of separate germs ; whilst the Red Snow is 
propagated by the latter only. It is interesting to trace, in a be- 
ing so extremely low in the scale, the two kinds of reproduction 
which are performed in a manner so much more complex, and 
apparently so different, in the higher plants. 

59. Of all the Cryptogamia, the Fungi are the most important 
to man ; and their influence seems at first sight exerted rather to 
his injury than to his benefit. Those minute species which con- 
stitute mildew, blight, rust, &c. often destroy to an immense 
amount the fruits of the earth upon which he relies for his chief 
support. An instance has been just recorded in which the lives 
of animals that administer to his luxury are also destroyed in large 
numbers. The decay of timber in the mode commonly termed 
dry-rot, is caused by the growth of Fungi, of which several spe- 



FUNGI. — DRY ROT. 57 

cies are frequently concerned in effecting this most injurious pro- 
cess. The ravages which they commit in ships and in every 
kind of wooden structure, as soon as a settlement is made, can 
only be conceived by those who have witnessed and examined 
them. The devastations they have committed on our navy and 
merchant vessels excited attention to the subject, and led to the 
invention of the process now known by the name of Kyanizing* 
(from the name of its inventor ;) but their destruction of house 
timbers is quite as rapid and complete, though less common. " I 
knew a house," says Mr. Burnett, " into which the rot gained ad- 
mittance, and which, during the four years we rented it had the 
parlours twice wainscotted, and a new flight of stairs ; the dry- 
rot having rendered it unsafe to go from the ground-floor to the 
bed-rooms. Every precaution was taken to remove the decay- 
ing timbers when the new work was done ; yet the dry-rot so 
rapidly gained strength, that the house was ultimately pulled 
down. Some of my books which suffered least, and which I 
still retain, bear mournful impressions of its ruthless hand; others 
were so much affected, that the leaves resembled tinder, and, 
when the volumes were opened, fell out in dust or fragments." 
The decay of the wood seems partly due to the growth of the fungi 
in its substance, which is decomposed by it, as are the fluid and 
half-solid matters already spoken of; and partly to the moisture 
which they are the means of introducing into its interior. The 
germs of these plants fall into the chinks of the timber, where 
they take root ; and in their growth, they greatly widen these 
chinks, and thus give admission to moisture from without, as 
well as to a new set of these minute germs, which may prove 
even more destructive ; and by a continuance and repetition of 
these processes, the whole strength of the timber is at last de- 
stroyed. 

60. The power of expansion which these plants possess, soil 

* This process consists in soaking the wood or other material in water 
in which corrosive sublimate has been dissolved; and in this manner a 
change is c fleeted which scchls to deprive the germs df Fungi of the power 
of obtaining nutriment. 
6 



58 USES OF THE FUNGI. 

as their tissues seem, is truly wonderful. Some years ago the 
town of Basingstoke was paved; and not many months afterwards 
the pavement was observed to exhibit an unevenness which could 
not easily be accounted for. In a short time after the mystery 
was explained ; for some of the heaviest stones were completely 
lifted out of their beds by the growth of large toadstools beneath 
them. One of these stones measured twenty-two inches by 
twenty- one, and weighed 83 lbs. ; and the resistance offered by 
the mortar which held it in its place would probably be even a 
greater obstacle than the weight. It became necessary to repave 
the whole town, in consequence of this remarkable disturbance. 

61. But though in these and many other ways Fungi are in- 
jurious to man, the benefits they confer upon him far outweigh 
their occasional devastations; and it is only through the con- 
stancy of the former that they are overlooked and unappreciated. 
It is not only to man that they are of the most essential service, 
but to the whole animal kingdom. To Fungi may be justly 
applied the designation which has been conferred upon Insects, 
that of the " scavengers of nature ;" for, like insects, they labour 
with the most astonishing effect in the removal of refuse and de- 
caying substances, which, were they left upon the surface of the 
earth, would prove not merely useless tenants but injurious in- 
cumbrances. Their vapour-like germs float about in the atmo- 
sphere in countless myriads, only waiting for the presence of a 
fitter soil on which to alight and grow. As long as there is no 
refuse decomposing matter to be removed, these spores remain 
inactive and undeveloped, (" the scavengers are unemployed ;") 
but as soon as any quantity, large or small, of decaying animal 
or vegetable matter, is left exposed, it is soon covered with a de- 
position of spores, which rapidly develope themselves into fungi 
of various kinds. 

62. Their astonishing fertility, and the rapidity with which 
they arrive at maturity, are among the most remarkable charac- 
ters of this tribe of plants. Of the former, some account has 
already been given. Of the latter many instances are recorded. 
Thus one species has been known to attain the weight of 34 lbs. 



IMPORTANT USES OF FUNGI. 59 

in six weeks; and on the continent, Fungi of this tribe have 
grown to upwards of 100 lbs., having begun from a point not 
perceptible to the naked eye. A large fungus of the Puff-ball 
tribe has been seen to grow in one night from a minute speck to 
the size of a large gourd. No other living beings have powers 
of growth at all to be compared to this. The more rapid the 
decomposition, and the greater the quantity of noxious exhala- 
tions which would thus be spread through the atmosphere, the 
greater is the tendency to multiplication and luxuriant growth 
in these humble plants, to which such exhalations serve as the 
most appropriate food. 

63. Hence what has been said by Naturalists of Insects, ap- 
plies with equal truth and force to these humbler tribes ; and we 
may adopt with slight modification an interesting statement 
which has been given of the agency of Insects, as a striking de- 
lineation of the operations of the Fungi. 

64. " The peculiarity of their agency consists in their power 
of suddenly multiplying their numbers, to a degree which could 
only be accomplished in a considerable lapse of time by any 
larger beings ; and then as instantaneous relapsing, without the 
intervention of any violent disturbing cause, to their former 
insignificance. If, for the sake of employing on different but rare 
occasions a power of many hundreds or thousands of horses, we 
were under the necessity of feeding all these animals at a great 
cost in the intervals, when their services were not required, we 
should greatly admire the invention of a machine, such as the 
steam engine, which should be capable at any moment of exert- 
ing the same degree of strength, without any consumption of 
food during the periods of inaction ; and the same kind of admi- 
ration is strongly excited when we contemplate the powers of 
Insect and Fungous life, in the creation of which Nature has 
been so prodigal. A scanty number of minute individuals, only 
to be detected by careful research, and often not detectable at 
all, are ready, in a few days or weeks, to give birth to myriads, 
which may repress or remove the nuisances referred to. But no 
sooner has the commission been executed, than the do-antic 



60 USES OF FUNGI. 

power becomes dormant ; each of the mighty host soon reaches 
the term of its transient existence ; and when the fitting food 
lessens in quantity, when the offal to be removed diminishes, 
then fewer of the spores find soil on which to germinate ; and 
when the whole has been consumed, the legions before so active 
all return to their latent unnoticed state, — ready, however, at a 
moment's warning again to be developed, and, when labour is to 
be done again, again to commence their work either in the same 
districts, or to migrate in clouds like locusts to other lands. In 
almost every season there are some species, but especially in 
autumn there are many, which in this manner put forth their 
strength ; and then, like the spirits of the poet, which thronged 
the spacious hall, * reduce to smallest forms their shapes im- 
mense.' " * 

65. Among the uses of Fungi to man, their service as food 
must not be forgotten. In chemical composition they more 
nearly resemble animal flesh than do any other vegetable sub- 
stances ; and, accordingly, those of them which are free from 
injurious properties furnish a highly nutritious article of diet, and 
some of the rarer species are greatly valued as dainties by the 
epicure. There is much difficulty, however, in distinguishing the 
innocent from the noxious species of Mushroom ;• and many 
fatal accidents have occurred from the employment of the poi- 
sonous kinds. Amongst the Fungi remarkable for their peculiar 
properties may be mentioned one which is of great service, from 
its astringent properties, as a styptic, to check the flow of blood ; 
and another which has the power, even when dry, of producing 
a curious kind of intoxication, and is used for that purpose by 
the Tartars, 



CHAPTER III. 

OF THE ELEMENTARY STRUCTURE OF PLANTS. 

66. When we examine yet more closely into the conforma- 
tion of the different parts of which an organized structure is com- 
posed, we find that, though the several organs are variously con- 
structed, and are adapted for different offices or functions, they 
are built up, as it were, of the same materials. With the same 
bricks, stones, mortar, and timbers, a church, a palace, or a pri- 
son may be reared. Just so it is in organized structures. We 
do not find that each organ is entirely different from the rest, 
though it has usually something peculiar to it ; but we are ena- 
bled to separate it into many distinct portions, something similar 
to which, if not exactly correspondent, may be recognised in 
other parts. Thus, for example, it was formerly stated that the 
leaf consists of a midrib and veins proceeding from it, a fleshy 
substance filling up the interstices, and a cuticle or skin cover- 
ing the whole. Now the midrib and veins, as well as the foot- 
stalk of which these are a prolongation, consist of three kinds of 
structure; — woody fibre, to which they owe their toughness, and 
by which they are adapted to give support to the softer struc- 
tures ; — ducts or canals for the transmission of fluid ; — and spiral 
vessels which are designed to convey air. On tracing these to 
the stem, it will be found that they all exist in it under the same 
form, and that these portions of the leaves are in reality but 
continuations of it. Again, if we examine the fleshy substance 
which lies amongst them, we shall find that it corresponds very 
closely in character with the pulp of soft fruits, or the pith 
of the stem. And if we strip off the cuticle and investigate its 
structure, we shall perceive that it is but another form o[' the 
substance, and that it corresponds with the skin which covers ail 

0* 



62 PRIMARY TISSUES OF PLANTS. 

the newly formed parts of the stem and branches, as well as the 
various parts of the flower, and even the roots. 

67. These several kinds of structure are termed the primary 
tissues, being the elements, as it were, of which the edifice is 
built up ; and they are to the vegetable fabric what the bones, 
muscles, fat, blood-vessels, nerves, skin, &c. are to the animal. 

68. Even these primary tissues may be regarded as consist- 
ing of other parts still more simple, — namely, membrane and 
fibre. The fleshy portion of the leaf, for example, or the pulp of 
fruits, consists of a number of little bags adhering together : each 
bag or vesicle consisting of a delicate membrane, without any 
perceptible orifice, and containing fluid. The membrane which 
encloses an egg after the shell is removed, will afford a good 
illustration on a large scale of the nature of these vesicles; they 
may, however, be readily distinguished and separated in an over- 
ripe orange, where they are of considerable size. Now the mem- 
brane which composes their walls may be regarded as one of 
the very simplest forms of vegetable tissue. Again, if the stalk 
of a strawberry or geranium leaf be carefully cut round but 
not through, and the two parts be then pulled asunder for a 
short space, a number of glistening fibres of extreme delicacy 
will be seen running from one portion to the other. If these be 
put under the microscope, it will be evident that they had lain in 
spiral coils, which are partially straightened when they are thus 
drawn out, just as when a spiral spring is strained. These were 
coiled within the membranous tubes that constitute the external 
sheath of the spiral vessels, which have been mentioned as exist- 
ing in the leaf-stalk ; and thus we are able to separate these ves- 
sels into the two other elements, membrane and fibre. These 
very minute delicate spiral fibres must not be confounded with 
the woody fibre, of which mention has been made, and the na- 
ture of which will be presently explained. 

69. The delicate Membrane, of which, in combination with 
fibre, all the tissues of plants may be regarded as consisting, when 
they are newly formed, is of variable thickness and transparency. 
In general, however, it is quite sufficiently transparent to allow 



ELEMENTARY MEMBRANE AND FIBRE. Od 

the colour of fluids in contact with it to be distinguished on the 
other side; and accordingly, though itself colourless or nearly so, 
it often appears tinged, in consequence of the cells or vessels 
which it forms being filled with coloured fluid. Thus, the cells 
of leaves appear green, those of the parts of flowers, yellow, blue, 
red, &c. ; not because that colour exists in the membrane of 
which they are composed (which, if they could be emptied, 
would appear almost colourless) but on account of the minute 
colouring particles diffused through their contained fluids. One 
of the most remarkable properties of vegetable membrane is its 
power of allowing fluids to pass slowly through it, even though 
no visible pores or apertures can be detected in it. Sometimes 
the appearance of such apertures exists, when membrane is 
highly magnified; but this appearance is sometimes produced by 
grains of semi-transparent matter sticking to it ; and is some- 
times due to that portion of the membrane being thinner than 
the rest, through the deposition of new matter upon certain 
points, subsequent to the first formation, of which several exam- 
ples will be presently given. 

70. Elementary Fibre may be compared to hair of extreme 
delicacy; its diameter often not exceeding the 1-12000 of an 
inch. It is generally transparent and colourless, and is usually 
disposed in a spiral direction. Its peculiar property is elasticity, 
combined, with a degree of firmness which, for its diameter, is 
very considerable. Accordingly we find its chief use to be the 
keeping open, like an interior spring, the delicate membranous 
tubes through which air is to pass, and the preventing these from 
being pressed together by the growth of neighbouring parts. IV ot 
unfrequently, however, it seems less elastic than usual, and is 
broken during the processes of growth into several smaller frag- 
ments, which then exhibit a peculiar tendency to grow together 
in various irregular forms. In this way several peculiar kinds of 
tissue are produced, which will now be noticed. 

71. The one most universally present, no kind of plant being 
without it in some form or other, and many being entirely com- 
posed of it, is that called cellular tissue, from its being made up 



64 



CELLULAR TISSUE. 



of a number of separate cells or minute bags adherent together. 
These, when first formed, are usually nearly globular, or of a 






Fig. 11. Various forms of cellular tissue; a, separate 
vesicles of an egg-shaped form ; b, section of cubical cellular 
tissue of pith; c, section of murif'orm cellular tissue, 



figure resembling an egg; so that, if cut across, their walls 
would exhibit a series of circles touching each other at certain 
points. Afterwards, however, they are gradually pressed against 
each other, and their sides become flattened. Their form will 
then depend upon the amount of the pressure on the respective 
sides. If it have been equal in all directions, the cell will some- 
times be cubical as it is often found in pith ; or it will have the 
ii ji .. form termed the dodecahedron, which is 

^OCXjfyinf a solid navin g twe l ve e q ua l sides. But 
JOOOOmi if it be pressed more on one side than 
)^)0(?POu6^ another, it will be narrowed in that di- 
C^rjQ^QQOC rection and elongated in the other. Thus 
SUOT^^C?^^ ^ e or ^ n al form of the cell may become 
WQ$t^£kd&QQDx greatly modified during the growth of 
the plant. In general, the greatest elon- 
gation takes place in the direction of 
most rapid increase; but this is not 

pressed cellular tissue; the always the case ; for in the stems of most 
honey- comb appearance of .»,.,. , . ,. 

the greater part is due to tre es in this climate, there is a peculiar 
the 12-sided form of the set of cells extending from the pith to- 
cells, the walls of which, , .< . , , . ... u ,, . , 

when cut across in any wards the bark ' which have their ^ reat * 
direction, present hexa- est length in a horizontal direction ; and 
gons or 6-sided figures. ^ ^ ^ Qf an oblong flattened 

form, and arranged like bricks in a wall, this kind of structure 
has been called muriform (wall-like) cellular tissue. 



mam 



Fig. 12. 
Section of irregularly com 



PROPERTIES OF CELLULAR TISSUE. 65 

72. From what has been said of the permeability, or power 
of giving passage to fluids, which vegetable membrane possesses, 
it may be inferred that this power is also possessed by the sim- 
ple modification of it just described. Accordingly we find this 
to be the case, — fluids being conducted through it very readily 
from one part to another : but still it affords a sufficient degree 
of resistance to cause the transmission of fluids most readily in the 
direction of the greatest length of the cells, where, of course, there 
will be the fewest partitions in a given space. Thus, therefore, 
fluids absorbed at the bottom of a stem, will pass upwards 
through its cellular tissue more readily than in any other direc- 
tion, except in the case of the muriform cellular tissue, which 
conducts fluids horizontally with the greatest readiness ; and the 
object of this peculiar adaptation is to convey the nutritious sap 
which is passing down the bark into the interior of the trunk. 
It will be more fully described in Chap. VI. where the structure 
and offices of the different parts of the stem will be severally 
detailed. 

73. In the fabric of the lowest tribes of Plants, such as Sea- 
weeds, Lichens, the Fungi (or Mushroom tribe,) Liverworts, and 
Mosses, little besides cellular tissue and its simple modifications 
can be found ; and it forms a large proportion of that of even the 
highest tribes. Thus in every Plant, the leaves, flowers, bark, 
pith, and fruit, consist almost entirely of cellular tissue ; and it is 
even found in the woody part of the stem and roots, besides 
forming the largest proportion of those soft succulent stems which 
are only of short duration, dying as soon as the fruit they bear 
has ripened. The whole of the young plant, too, even of the 
highest tribes, consists, like the permanent forms of the lower, of 
this kind of structure. It is only when the true leaves have been 
unfolded and are actively performing their functions, that the 
other kinds of tissue show themselves. In all newly-forming 
parts, also, the foundation, as it were, is laid with this tissue, in 
which the others subsequently appear. So universally is it 
present, even in the adult fabric, that, if it were possible to ab- 
stract all the others from it, the original form would still be re- 
tained, except where it would give way with its own weight. 



66 



MODIFICATIONS OF CELLULAR TISSUE. 



74. But, although cellular tissue is, in its regular state (of 
which the pith of young twigs, or the pulp of fruits are characte- 
ristic examples) soft and spongy in its charac- 
ter, it does not always remain so, but often 
acquires considerable hardness. This is the 
case, for example, in the prickles of the Rose 
and other plants, which are merely connected 
with the cuticle and are not prolonged from 




Fig. 13. 



fnTd^by^nlelnai the W °° d beneath ' lt is the ca * e a]s0 in the 

deposites arranged stones of the Plum, Peach, Cherry, &c. ; and in 

CIS 11 "' 7 ^ Clr * the gritty matter in the centre of the Pear - 
In all these parts, the processes of vegetation 

are no longer going on; but 
the power of firm resistance is 
required in their place. This is 
effected by the deposition of 
solid matter within the cells. 
Sometimes the new product 
lies in regular layers, one with- 
in another, covering the whole 
membrane ; sometimes it is de- 
posited in what appears a less 
regular manner, certain points 
of the membrane being left un- 
covered by it. In this last case, 
i4 

Sections of cells 'strengthened by however, an additional object is 
internal matter irregularly de- attained; for the cells, though 
posited; the shaded portion indi- , _ f t n _ rf f th • 11 • 

cates the remaining cavities: a, the gl eatest part Ol their walls IS 
cells from the gritty centre of the so much thickened, are still in 
peay&, cells from the stone of the a degree permeable to fluid , 

through the spots of the mem- 
brane on which no deposite has taken place. These spots in 
the walls of contiguous cells generally correspond with each other ; 
so that fluids can find their way from one cell into the cavities 
of the neighbouring ones ; though so large a proportion of their 
contents has become solid. When the walls of cells have been 
thus strengthened in particular parts, the membrane has a doited 




WOODY FIBRE. 



67 



appearance, the thinnest portions seeming almost like perfora- 
tions. 

75. The size of the cellules of this tissue is extremely variable ; 
they are usually from 1-300 to 1-500 of an inch in diameter ; but 
may be found of all sizes, from 1-30 to 1-3000 of an inch. One 
of the most interesting modifications of it is found in the Sphag- 
num or Bog-Moss ; and in the coverings of some seeds. This 
consists in the presence, within the membranous wall of the cell, 
of a spiral fibre, coiling from one end to the other. In some of 
the seed-coats in which these spiral cells exist, the membrane of 
the cells is so delicate as to be easily dissolved away ; so that, if 
a portion be put into water, the fibres spring out very beautifully 
by their own elasticity. 

76. The next form of elementary tissue to be described is that 
called Woody Fibre. It has received the name of fibre, 
because it always exists in an elongated form, and se- 
veral of the tribes of which it consists adhere together 
continuously so as to form cords. This is seen in the 
common flax thread, for example. If the finest thread 
that could be separated with the naked eye were sub- 
mitted to a microscope, it would be seen to consist of 
several other fibres adhering together; none of these have 
any great length ; but by the manner in which they ad- 
here, side by side, and end to end, a continuous cord is 
produced. Each of these minute fibres, when more 
closely examined, is seen to consist of a slender trans- 

Fi<r. 15 . parent tube, tapering to a point at each end. It thus re- 
Bun dlu sem bles a greatly elongated cell. It differs from cellular 
of Woody 5 J to , , ■-- 

Fibie. tissue of similar form, in the much greater strength of 

the membrane forming the walls of the tubes, though it is at the 
same time thinner. There are many intermediate forms, however, 
between one and the other. Woody fibre is evidently destined to 
convey fluid in the direction of its length, and is easily permeated 
by it. Minute openings have sometimes been detected in the points 
of the tubes, so as to connect one cavity with another, and thus 
to render the passage of fluid more easy. It is, however, especially 
destined to give firmness and elasticity to the parts of the stTUO- 



68 



MODIFICATIONS OF WOODY FIBRE. 



ture which require support ; and we almost constantly find ves- 
sels protected by it wherever they exist. 

77. In all plants with permanently-elevated stems, this tissue 
is very abundant in the adult state. It forms a large proportion 
of the wood of the stem and roots ; it partly composes the leaf- 
stalk, midrib, and veins of the leaves, and may even be traced in 
flowers ; to many fruits, also* it imparts firmness and consistence. 
When no longer required for the conveyance of fluid, additional 
firmness and toughness are given to it, as to cellular tissue, by 
the deposition of various secretions within its tubes ; and it is in 
the presence or absence of these that the difference exists between 
the heart-wood and sap-wood of a trunk. The woody tubes of 
the former are entirely choked up with the hard matter deposited 
in their cavities ; and the sap rises through the latter only. This 
hardened tissue may be in some degree compared to the carti- 
lage or gristle of animal bodies. 

78. A peculiar form of woody fibre is found in the stems of 
resinous woods, especially the Pine and Fir tribe. The diameter 
of its tubes is much greater than that of any other woody tissue ; 
and they alone perform the office of transmitting the sap upwards 
through the stem, the wood of these trees being destitute of the 
ducts or canals (presently to be described) which in other kinds 
of trees assist in this function. But it is by a peculiar set of dots 

seen along their course, that these woody 
tubes may be readily distinguished from 
all others. These dots appear to be 
formed by the adhesion of some little 
bodies to the interior of the tube, or by 
their growth on the exterior so as to pro- 
Glandular Woody Fibre of J ect int0 the cavit y b y the Pressure of 
a Deal Shaving. two adjacent tubes. Whatever be their 

character, which is not yet certainly ascertained, they are of 
great interest, as aiding to establish the true nature of coal. 

79. That this substance had a vegetable origin has long been 
generally admitted ; but from the comparative frequency and per- 
fection with which the remains of Ferns occur in it, it has been 
supposed to have been produced by the decay of vast forests of 



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4 



ORIGIN OF COAL. SPIRAL VESSELS. 



69 



this tribe of plants. As Ferns do not form resins, however, this 
hypothesis would not account for the large quantity of bituminous 
matter which coal contains ; and hence it was supposed that coal 
must have been formed from resinous woods, even though the 
remains of such were very scanty and imperfect. Now on ap- 
plying the microscope to transparent sections of such fragments 
of coal as most distinctly exhibit the fibrous structure, it is seen 
that they present the character which has been described as pe- 
culiar to the resinous woods, — the glandular form of woody fibre, 
as it is technically termed ; and hence it is established beyond 
doubt, that the immense masses of coal which now contribute so 
much in every way to the comfort and the social improvement 
of the human race, are but the remains of vast forests, probably 
the growth of many successive centuries, which chiefly if not en- 
tirely consisted of trees of the Pine and Fir kind. It is even pos- 
sible, by the peculiarities of the arrangement of the dots, to say 
which of the subdivisions of that tribe at present existing, those 
primeval trees most nearly resembled. 

80. The third kind of primary tissue is that de- 
nominated the Spiral Vessel, with its modifica- 
tions. Its essential character is the possession of a 
spiral fibre coiling within its thin membranous 
tubes from one extremity to the other. This fibre 
is not always to be traced, however, with the 
same regularity. The true spiral vessel much re- 
sembles the woody fibre in form, being a long nar- 
row tube drawn to a point at both ends. But the 
membranous wall is much thinner, and is easily 
torn asunder. The spiral filament is usually 
single ; it is sometimes, however, double, or even 
triple; and in the very large spiral vessels of the 
Portions of Spi- Chinese Pitcher-plant (Nepenthes \ 242,) it is 
common form] quadruple. These tubes in their perfect state 
with single fibre contain air only, which finds its way from one to 

partly drawn u nke fl id through the WOOdy tubes. 

out; 6, from JNc- y 

pontiles, with They are found in the leaf-stalks, from which 

quadruple fibre. theh . gpira j fibres (>an be unco[]cd in ihc manner 




70 SPIRAL VESSELS. AIR-TUBES OF INSECTS. 

already described. They are found also in a delicate membrane 
surrounding the pith of those which possess one (§ 135 ;) and in 
the midst of the woody bundles which form the strings of such 
stems as the Asparagus. From this plant, indeed, they may be 
obtained more readily, perhaps, than any other. If a stem be 
boiled, or softened by soaking in water for some time, and these 
bundles be separated from the soft tissue which surrounds them, 
the parts of each may be farther separated from each other by rub- 
bing them, with a little water, between two plates of glass. On 
looking at them with a magnifying glass, some portions of these 
bundles will be seen to present a dark appearance if still under 
water. This is caused by the air they contain ; since bubbles 
of air in fluids viewed with the microscope, will appear dark to 
the observer, for reasons which will be mentioned in the Treatise 
on Light. If one of these threads be then carefully torn, with a 
pair of small needles fixed in handles, into finer ones, whilst under 
a powerful single magnifier, it may be separated into the indi- 
vidual spiral vessels which compose it, just as the thread of flax 
may be resolved into its woody tubes. 

81. It is an interesting circumstance that the air-tubes of In- 
sects are formed upon nearly the same plan with these spiral 
vessels of plants. The former consist, like the latter, of an ex- 
ternal membrane, which is maintained in its tubu- 
lar form, in spite of pressure from without, by 
the elasticity of a fibre spirally coiled in its interior. 
The principal difference between the two struc- 
tures is, that the air-tubes of plants are closed 
vessels, and that their contents find their way 
gradually from one to another, permeating the 

Branching Air- delicate membrane of their walls ; and that they 
vessel of Insect, give off, therefore, no branches : whilst the air- 
vessels of insects, whose office it is to convey air with great ra- 
pidity into all parts of the structure, form a set of continuous 
tubes, which branch and ramify with the most wonderful mi- 
nuteness even in the smallest organs of the smallest Insect. 

82. The other kinds of tissue which we find in plants may be 




STRUCTURE OP DOTTED DUCTS. 



71 




Fig. 19. 



regarded as modifications of the foregoing ; but still they are suf- 
ficiently distinct in their character and offices to require a sepa- 
rate consideration. The first of these consists of the long straight 
non-branching canals which are found in the stems and leaf-stalks 
of the higher kinds of plants, and are termed Ducts. These appear 
to be originally formed by the breaking-down of the partitions 
between large cells that were laid end to end, so that a continuous 
tube is produced , for in many instances the remains of such 
partitions can be detected. Sometimes these ducts remain, like 
the cells from which they originated of a simply 
membranous character ; but more commonly their 
walls are fortified by an interior deposite, which 
does not, however, entirely line them, but leaves 
the membrane bare at certain points, giving that 
dotted appearance already described in treating of 
the cells. Hence these vessels are commonly termed 
dotted ducts. It is through such as these, that the 
Section of a sap principally rises in the stem and branches, and 
Duct^how* is conve y ed to the leaves. They are by far the 
ing that the largest vessels contained in the vegetable fabric, and 

otsare ; their open mouths are visible in almost any stem 
ner spaces of r J 

its walls. when cut across. They are of particularly great 

diameter, when the stem itself is small 

and long, but bears a considerable 

amount of leaves, as is the case in the 

Vine and the common Cane; in 

these, their orifices at once strike the 

eye ; and, if the stem of a growing 

Fig. 20 - plant be cut across, the oozing of the 

A bundle of such ducts cut , 

across ; o, a, pits in their sides sap from their mouths w 
corresponding with pits in the distinguished, 
adjoining cells ; b, a depression 
without a corresponding one *° 
on the other side. Ducts, which seem to belong rather 

to the third form of Elementary tissue— the class of Spiral Ves- 
sels ; since they exhibit more or less perfect traces of the presence 
of a spiral fibre within the membrane of the tube. These are the 




be easily 
But there are other kinds of 



72 



SPIRAL AND ANNULAR DUCTS. 



cases in which the fibres show the greatest want of elasticity. In 
Ferns we find Ducts which very closely approach the spiral vessel 
in character ; having an unbroken coil of spiral fibre throughout 
their whole extent ; but, besides the important difference that 
these Ducts are long continuous tubes, they are farther dis- 
tinguished by the brittleness of the spire, which snaps if we 
attempt to unrol it. Such ducts are found in many other plants, 
and may be easily distinguished in the leaf-stalk of the Rhubarb. 
Now there are others in which we see an irregular spiral, — the 
jl&j coil sometimes terminating in a ring, and then com- 
mencing again with perhaps the intervention of 
two or three rings. Here it would seem as if the 
membrane had grown faster than the spire could 
follow it; so that the fibre, not being elastic, had 
been occasionally broken. In other ducts, again, we 
find no traces of a spiral fibre ; but the membranous 
walls are distended by rings at intervals sometimes 
tolerably regular. These are called annular or 
ringed vessels. 

84. These two forms are especially interesting 
from the analogies which can be found to them in 
the animal structure. The close correspondence 
between the spiral vessels and the tracheae or air- 
tubes of insects, has been already pointed out. On 
the other hand, the annular duct corresponds with 

t^ I s "... the wind-pipe of higher animals, the membranous 
Ducts with * v & ' 

internal fibre; walls of which are prevented from falling together 

b, spiral, with ^y means f rings of cartilage disposed at regular 
rings at inter- J ° & r & 

vals; a, annu- intervals. And the half-spiral half-annular duct 

lar through. w hj cn is the intermediate form in plants, precisely 

corresponds with the structure of the wind-pipe of 

the Dugong (one of the Whale tribe) in which we find a spiral 

cartilage terminating at intervals in rings. 

85. There are other forms of ducts, again, in some parts of 

which the traces of the spiral structure are very obscure ; whilst 

in other portions of the same tube they can be easily distinguished. 



SPIRAL DUCTS. BRANCHING SAP- VESSELS. 



73 



In these it appears as if the spiral fibre had been broken up into 
small fragments, and that these had served as centres round which 
new deposites had accumulated, so that they had grown irregu- 
larly together, leaving interspaces in which the membrane is un- 
covered (as in the dotted duct) by this secondary wall. In fact 
it often happens that a duct, which exhibits in one part distinct 
remains of the spiral structure, approaches the character of the 
dotted duct so closely in another part, that they can scarcely be 
distinguished ; and it is probable that the interior deposite which 
gives to the latter its peculiar character may have originally 
taken place around the fragments of a spiral fibre. 

86. The office of all these ducts is the same, — that of convey- 
ing fluid. It is only in the true spiral vessel that we find air. 
These varieties have been described with somewhat greater mi- 
nuteness than may appear necessary ; because the young observer 
who examines the vegetable structure for himself, as it is hoped 
that many will be led by these pages to do, will be liable to be 
perplexed by meeting with them if not previously acquainted 
with their characters. 

87. One other form of elementary 
tissue now remains; and this differs 
from all the rest. It is a system of 
branching vessels, confined to the 
under side of leaves, and to the bark, 
and serving only for the conveyance 
of the nutritious sap, which is carried 
by it from the leaves where it is pro- 
duced, down the bark and thence to 
all parts of the structure. The walls 
of these branching vessels are ex- 
tremely delicate, so that they can be 
scarcely separated from the tissue 
around ; hence it was long supposed 
that the nutritious sap, or proper 
juice as it is generally termed, flowed 
in more spaces amongst other tissues, 
7* 




Fig. 22. 
System of branching vessels 
for the conveyance of the lit lex 
or nutritious juiec. 



/ 4 STRUCTURE OF THE CUTICLE. 

and not in distinct tubes. The existence of these, however, is 
now well established ; and there can be little doubt that, like the 
straight ducts, they take their origin from cells, the partitions be- 
tween which are broken down, so as to form a complete network 
of canals. 

88. In future chapters, the combinations of these tissues in 
the several organs, such as the Stem, the Leaves, the Flowers, 
&c. will be described; but it. may be well here to speak of one 
peculiar modification of cellular tissue which is seen in all these 
parts, — that, namely, which forms the cuticle or skin in which 
they are enveloped. The existence of this is easily shown in 
many leaves without preparation. From the leaf of the common 
garden Iris, for example, it may be easily stripped, or from the 
under side of that of the London Pride ; and from every leaf it 
may be easily removed after being soaked for a few days in wa- 
ter. This cuticle is found to be usually transparent and nearly 
colourless. If when separated it should appear coloured, this is 
due to the adhesion to it of some of the cellules of the fleshy por- 
tion (or parenchyma) of the leaf; these will afford an opportu- 
nity of examining the form and structure of these cellules ; and 
they may then be wiped away, leaving the membrane perfectly 
smooth and colourless on both sides. Now when this is examined 
with a sufficient magnifying power, it is seen to consist of a num- 
ber of flattened cells in close contact with each other ; and these 
cells contain no fluid, but are filled with air. Their form is 
very different according to the kind of plant examined. Some- 
times they are of a regular oblong and their sides straight ; whilst 
in other instances they are of very irregular form, and lock into 
one another like the pieces of a dissected map. 

89. Though the cuticle usually consists but of one layer of 
cells, it sometimes contains tivo or even three, especially in plants 
naturally growing in warm climates ; and in the Oleander four 
may sometimes be distinguished. Its office appears to be to 
prevent the moisture of the soft succulent tissues beneath from 
evaporating ; since, if they were to dry up, their vital properties 
would be lost. Accordingly we find it absent in plants which 



CUTICLE. STRUCTURE OF STOMATA. 75 

habitually live beneath the surface of the water ; and from those 
parts of others which are usually submerged ; whilst it is pre- 
sent on those parts of the same plants which are lifted into the 
air, as well as on all the soft parts of those which are habitually 
and entirely exposed to it. Its use is at once seen when a por- 
tion of a plant destitute of it is exposed to the air ;— it speedily 
dries up and withers. On the other hand, the Oleander, exposed 
to the intense sunshine of tropical Africa, maintains its verdure, 
even in arid situations, by the great resistance to evaporation 
which its thick and almost leathery cuticle interposes. The best 
mode of separating this cuticle, so as to become acquainted with 
its remarkable firmness, is to soak a leaf for a few days in water 
rendered sour to the taste by a few drops of nitric acid ; and it 
may then be easily stripped off. But its different layers can only 
be seen by magnifying a very thin slice of the leaf cut across, so 
that its thickness, not its surface, is exposed to view. 

90. The entire of the softer portions of all plants growing in 
air is covered by cuticle ; and in the young plant the whole sur- 
face. It is only when the stem increases in diameter, and the 
bark becomes hard and rugged and occasionally scales off, that 
the cuticle can no longer be distinguished. It is evident on young 
shoots as on the leaves, and may be traced downwards to the 
point of the root ; but this it does not cover. It also protects all 
the organs of which the flower is composed ; but it is absent at 
one point, for reasons hereafter to be stated. 

91. The tissues protected by the cuticle are not entirely cut 
off by it, however, from the external air ; for it has certain aper- 
tures of a very peculiar character, which open or close under the 
influence of light. The apertures are called Slomata (mouths.) 
They are usually of an oval form, and are bounded by two kid- 
ney-shaped cells containing green matter ; and it is by the ex- 
pansion or contraction of these that the orifice is diminished or 
increased. Sometimes, however, the opening is round, and is 
bounded by a ring of four or live such cells ; and in the very 
curious stomata of the Marchantia pohpnorpha, one of the com- 
monest of the Liverwort tribe (\ 3:2,) there are live such rings, 



76 



STRUCTURE AND SITUATION OF STOMATA. 



one beneath the other, the aperture resembling a funnel, and the 
lowest ring being the one which regulates the amount of com- 
munication between the chamber into which it opens, and the ex- 
ternal air. 




A B C 

Fig. 23. Views of Stomata. A, vertical section of stoma 
of Iris ; a, a, green cells bounding the orifice ; b, &, cells 
of the parenchyma ; c, air chamber. B, view of the same 
from above; a, a, green cells of the stoma, lying between 
long cells of the cuticle; c, opening between them. C, 
1 similar view of a stoma of apple-leaf; a, cells of the 
stoma; b, b, cells of the cuticle ; c, opening of stoma. 

92. Stomata are always placed over interspaces in the tissue, 
which are called intercellular passages ; they are never found 
on the midrib or veins of a leaf, nor in fact over any hard woody 
portion of the structure. They are chiefly disposed over the soft 
green tissue of leaves and young shoots; but they are found also 
on the parts of the flower. When the leaves are absent, and the 
stem performs their functions, — as in the Cactus or Prickly-pear 
tribe, — stomata are found on its surface. They are generally- 
most abundant on the under surface of leaves, and are sometimes 
altogether absent from the upper. This is partly due to the fact 
that the tissue lying beneath the upper surface of leaves is so 
closely packed together, that there are scarcely any intercellular 
passages, into which the stomata might open ; whilst the tissue in 
contact with the lower cuticle is extremely loose in comparison, 
and abounds with such passages; hence it is that the colour 
of the upper surface of the leaf is usually so much deeper than that 
of the lower. But in leaves of which the two sides are equally 
exposed to the air and light, such as those of the Iris and of the 
common Flags growing by the sides of brooks, the general struc- 
ture is nearly the same on the two sides, and the stomata are 
equal in number. Again, in Plants, the circumstances of whose 
growth are such that the atmosphere commonly comes in contact 



OFFICE AND FORMATION OF STOMATA. 77 

with the upper side only of the leaf, — as in the case of the Water 
Lily, the leaves of which float on the surface of the water, — the 
stomata are disposed on that side alone. 

93. As there is no cuticle to protect the tissues of plants grow- 
ing altogether beneath the surface, so there is no occasion for 
stomata to admit the passage of air to these ; and accordingly in 
the whole tribe of Sea-Weeds we find no vestige of them. Nei- 
ther can they be distinctly traced in the Mushroom-tribe, nor in 
Lichens ; but in the Liverworts they present themselves in the 
most remarkably complex form which we any where witness ; 
in the Mosses they have only been detected on the stalk which 
bears the fructification ; whilst in most Ferns, as well as in Flow- 
ering Plants, they abound. 

94. Of the very minute size of these curious organs, some 
idea may be formed from the fact that in some leaves it is esti- 
mated that 70,000 occur in a square inch of cuticle. The largest 
known are about the 1-500 of an inch in length; whilst the 
smallest are not 1-3000. Their function is evidently to allow of 
that limited evaporation of water from the soft tissues of the plant 
which will hereafter be shown to be one of the most important of 
the processes by which the crude fluid absorbed by the roots is 
converted into the nutritious sap or proper juice. The influence 
of light upon the stomata causes them to open, whilst they con- 
tract and even close in darkness. 

95. It has also been shown that light has a most important 
influence on their first production. In the young plant of the 
Marchantia (§ 33,) when first separated as a kind of bud from its 
parent, no stomata or roots exist. It has been ascertained by 
repeated experiments, that stomata and roots may be caused to 
develope themselves in either of the two sides, the stomata being 
always formed on the upper surface, under the influence of light, 
and the root-fibres proceeding from the lower towards darkness. 
But if the surfaces be reversed after the respective organs have 
been developed to a certain point, so that the stomata be directed 
towards the ground, and the roots be made to rise into the air, 
the little plant will right itself by twisting itselfround, so as to bring 
its surfaces to their former position. Farther, when plants o[ a 



78 



APPENDAGES TO THE CUTICLE. HAIRS. 



higher description are grown in darkness, the stomata are de- 
veloped very imperfectly or not at all. Thus, we have an 
example of the very important effects of the stimulus of light 
upon the vegetable structure, not only in governing its actions, 
but in influencing its development. 

96. With the cuticle may be advantageously considered 
those appendages which are developed from it, as hairs, prickles, 
stings, &c. The leaves and stems of many plants are covered 
with hair, which is sometimes bristly, sometimes soft and downy, 
and sometimes scattered very thinly. The structure of these 
hairs is various. Sometimes each forms but one long cell ; whilst 
in many other instances, every hair consists of a row of cells 
placed end to end, and sometimes these send off minute side 
branches. The cells of the hairs are usually, like those of the 
cuticle, destitute of fluid contents, except during the period of 
their formation. Their analogy with those of the cuticle is far- 
ther shown by the curious fact, that many plants are hairy or 
not according to the circumstances in which they grow. Thus, 
when they are found in dry exposed situations, their stems 
stunted in growth, and their leaves small, their surface is covered 




Fig. 24. Hairs and Glands of various kinds ; a, gland surmounted 
by a hair; 6, small gland at the top of a hair; c and e, simple hairs; 
d, branching hair. 



STINGS, PRICKLES. AIR-CHAMBERS. 



79 



with hairs, as if the cells which would have otherwise formed a 
larger cuticle had taken the shape of hairs: whilst in damp shady 
places, which favour the extension of the leaves and stems, their 
surface is quite smooth, all the material being then required to 
form cuticle. 

97. Sometimes the hairs are tubular and pointed, and are 
fixed upon minute glands in the cuticle which secrete an acrid 
fluid ; and if but very slightly touched, the reservoir at the base 
is compressed, and the fluid forced up through the tube into the 
wound made by its pointed extremity. Such hairs are termed 
stings; and the Nettle affords a familiar example of them. The 
prickles of the Rose and other shrubs are also appendages of the 
cuticle, with which they are stripped off, and from which it is 
easy to detach them. They are thus distinguished from thorns, 
which proceed from the wood of the branch, and which, as will 
be hereafter stated, may be regarded as stunted leaf-buds. 
Prickles, after being once formed, and hardened by the process 
already described (§ 74,) undergo no subsequent enlargement ; 
and, accordingly, if the cuticle on which they are fixed should be 
extended, their base is not able to expand in the same proportion, 
and they drop off. 

98. Another interesting modi- 
fication of cellular tissue is that 
which surrounds the spaces or 
cavities formed in certain plants 
for special purposes. Thus in the 
Duckweed, the leaves are provided 
with a set of air chambers, which 
give them great buoyancy ; and 
nothing can be more beautiful than 
the manner in which the walls of 
Fig 25. Air chambers of aquatic tl chambers are built up of 

plant, the vertical walls formed of * 

muriibrm cellular tissue a, a, hori- muriform cellular tissue. In other 
zontal partitions. cases> these cavities appear to be 

formed as receptacles for certain secreted products ; and here, 
too, they are very beautifully partitioned off from the surrounding 




80 



RECEPTACLES FOR SECRETION. 




Fig. 26. Receptacle for 
fluid secretion ; a cavity- 
bounded by cellular tissue. 



tissue, by a peculiar disposition of the 
cells. A good illustration of these is 
found in the rind of the orange and 
lemon ; the odour and flavour of which 
are derived from the minute drops of 
volatile oil stored up in a vast number 
of these little cavities. The turpentine 
of resinous woods is collected in larger 
channels of the same description. 
99. It is scarcely possible to observe 
the number of different forms (of which many have been left un- 
noticed) resulting from the varied combinations of the simple 
elements — membrane and Jibre, — each of them probably having 
its peculiar function in the vegetable economy, without being 
struck with the simplicity of the plan by which Creative Design 
has effected so many marvels, as well as with the extreme beauty 
and regularity of the structures which are thus produced. The 
comparison of such specimens of Nature's workmanship as the 
meanest plant affords, with the most elaborate results of human 
skill and ingenuity, serves, only to put to shame the boasted 
superiority of man ; for whilst every additional power which is 
applied to magnify the latter serves but to exaggerate their de- 
fects, and to display new imperfections, the application of such to 
organized tissues has only the effect of disclosing new beauties, 
and of bringing to light the concealed intricacies of their struc- 
ture. 



We shall next pass on to consider the structure of the com- 
pound organs of the Vegetable fabric, and their several purposes 
and uses. 



CHAPTER IT. 

STRUCTURE AND FUNCTIONS OF THE ROOTS. 

1 00. The Roots are the parts of the plant on which it is chiefly 
dependent for the supply of the moisture which its growth re- 
quires ; and they also serve to fix it in the earth. That they ab- 
sorb or suck up fluid with great rapidity may be easily shown in 
the following manner. Take any small plant that is growing in 
the soil, and immerse its roots (first clearing them of earth) in a 
tumbler of water. If the plant be exposed to the light of day, 
and especially if the sun shine brightly upon it, the water will 
disappear very much faster from the glass, than from one exposing 
the same surface, and placed in the same circumstances, but with- 
out the plant. And if the specimen continue to grow and flourish, 
it will take up many times its own weight of water in a short 
period. Thus, four plants of Spearmint, grown during 56 days 
with their roots in water, and themselves weighing altogether 
but 403 grains, have been observed to take up 54,000 grains, or 
about seven pints of the fluid. 

101. Of the water thus absorbed, a small proportion only is 
retained within the plant, serving as a part of its food. The 
greatest part of it is sent off again from the leaves, by a process 
hereafter to be described, termed Exhalation ; and the rapidity of 
Absorption is in part governed by the rapidity of Exhalation. 
The latter is nearly checked by the absence of light ; and, accord- 
ingly, plants are found to absorb but little in the night, or in a 
dark room. Any of the causes which will be subsequently stated 
to influence the latter, affect the former in a nearly corresponding 
proportion. The object of the introduction of a quantity of fluid 

8 



82 ABSORPTION IN THE CRYPTOGAMIA. 

into the vegetable system, so much larger than it retains, appears 
to be this ; — the solid mineral matters which constitute an im- 
portant part of the food of the plant, are contained in the water 
which reaches its roots in excessively minute proportion; and it is 
therefore necessary, in order that a sufficient amount of these 
may be obtained, that all the water in which they are dissolved 
should be introduced. As this is by far too much for the other 
wants of the plant, a large part of it is got rid of by Exhalation. 

1 02. It is only in the more perfect plants, however, that we 
find a restriction of this power of absorption to one particular 
portion of the structure. In the Sea- weeds, for example, the 
whole surface appears equally endowed with this faculty ; and 
there is, therefore, no occasion for that transmission of fluid from 
one part to another, which is characteristic of those in which we 
find a stem (or something correspondent with it) bearing roots at 
one end, and leaves at the other. Accordingly, that which is the 
natural condition of the latter, is fatal to the stemless plants; for 
if a Sea- weed be suspended partly out of the water, the upper 
portion will die from drought, whilst that which remains im- 
mersed will continue to live and grow, without transmitting any 
of its moisture to the rest. Not unfrequently we observe the 
form of a stem and roots in Sea- weeds; but this is only for their 
attachment to rocks, or other solid substances ; and the root-like 
portion has no special power of absorption, nor the stem of 
transmission. 

103. In some of the Cryptogamia a little higher in the scale, 
however, we find a condition much more approaching that of 
the Flowering-plants. Thus, in the Mushroom, we observe a soft 
stem, which sends off fibres into the ground ; and these appear 
to absorb by their whole surface, and to transmit the fluid they 
have acquired to the portion which is elevated in the air. In the 
Mosses, the tissue of the stem is firmer, and the rootlets which 
proceed from it are more woody ; these not only proceed from 
the stem but also from the under surface of the leaves ; and thus 
the dryness of the situations in which these interesting little 
plants find their subsistence is in some degree compensated for. 



FORMATION OF SPONGIOLES IN FLOWERING-PLANTS. 83 

In Ferns we have a woody stem and widely ramifying roots, like 
those of the Phanerogamia. 

104. If we examine the roots of any common plant with a 
branching woody stem, such as the Rose, we shall find that they 
subdivide and spread beneath the ground, very much upon the 
same plan with the branches above. Moreover, it will be seen that, 
from the sides and extremities of these underground branches, 
there proceed a number of delicate fibres ; and if the extremities 
of these be carefully examined, they will be found to be much 
softer than the rest of the structure. Now these fibrils are the 
true roots ; and their soft succulent extremities, which are called 
spongioles, are the parts by which alone they absorb or suck up 
fluid. This is easily proved. If a growing Radish be carefully 
removed from the ground, and the fleshy portion be bent in such 
a manner that it can be covered with water, whilst the leaves and 
tuft of fibres at its point are not immersed, it will be found that 
the whole of this large surface does not absorb moisture enough 
to keep the plant alive. But, on the other hand, if this tuft of 
fibres be only so far immersed in the water that their points may 
touch it, whilst the rest of the root is above the surface, the plant 
will continue to flourish. 

105. The fact is that this absorption takes place with the 
greatest rapidity through soft newly-forming tissue ; and this is 
what gives the spongioles their peculiar power. They are, in fact* 
the growing points of the rootlets, which are constantly increasing 
in length, and which in this manner go in search, as it were, of 
the supplies of food of which they have exhausted the soil that 
previously covered their extremities. As this growth continues, 
the tissue at first formed gradually becomes consolidated ; and 
when it has become hardened, it is no longer adapted for absorp- 
tion in more than a very trifling degree ; so that to the newly- 
forming point of the fibre this power is always nearly restricted. 
But in the young plant there is an interesting correspondence 
with the condition of the lower tribes ; for the soft roots, which are 
first sent down from the seed when it is commencing to grow or 
germinate, are, like the fibres proceeding from the base of the 



84 



FUNCTION OF THE SPONGIOLES. 



Mushroom-stem, capable of absorbing by their whole surface ; 
and it is only when woody fibre begins to be formed in them, 
that the power is restricted to their extremities. 

106. The knowledge that the delicate fibres, proceeding from 
what are commonly known as roots, are the true and only organs 
of absorption, has an important practical application. It very 
commonly happens that in transplanting shrubs or trees, of which 
the roots extend a good way into the soil, enough care is not taken 
to preserve these; and the plant either languishes for some time, 
until it is able to form new ones, or dies altogether. It is seldom 
that, under common treatment, a fruit tree will bear in the first 
season after transplantation. The following instance, however, 
will show what mode of proceeding is directed by the knowledge 
just communicated, and what success attends it. A gentleman 
in Shropshire had some valuable vines, which he wished to re- 
move to a new property on which he was going to live in Nor- 
folk. He had a trench dug around them, at such a distance as, 
it was believed, would include all their roots. The earth within 
this was then removed, not with spades and trowels, but with the 
fingers, — every fibril being thus uncovered without injury. The 
mass of roots was then wrapped in moist matting ; the vines 
were carried across England, and then planted ; and in the en- 
suing season they bore an abundant crop. 

107. It is often observed that the growth of roots takes place 
in the direction best adapted to supply them with moisture ; and 
it has been supposed that plants possessed a kind of instinct, or 
consciousness, which caused them to select this. Many of these 
cases, however, can be explained without having recourse to such 
a supposition; and it is probable that, with the advance of know- 
ledge on this subject, the remainder will be also. In fact, to attribute 
such an instinct to plants is to place them above the lower animals, 
which do not exhibit the power of making a choice of this kind. 
The most common cases are those most easily explained. A plant 
is in a dry soil, and it sends out its roots into a moister one ; or it 
it is in a garden pot, and its roots project through the hole at 
the bottom, into the water which the pan below it may con- 



DIRECTION OP GROWTH OF ROOTS. 85 

tain v or into the moist earth with which it may be surrounded. 
Now this is explained upon the following simple principle. The 
addition which is constantly being made to the extremities of the 
fibres, takes, place in the direction of least resistance; and, when 
the roots are making their way through a hard dry soil, the di- 
rection of least resistance will be that in which the earth is. 
loosened by the flow of water; towards the source of that 
moisture, therefore, the growth of the roots will be directed. 

108. The same principle has another curious application. 
Roots have been known to insinuate themselves into the crevices 
of walls, or even into chinks in the stones themselves ; and to 
burst asunder the walls of these after some time. Now when a 
root meets with such an obstacle as this in its growth, it is turned 
aside for a time, and endeavours, as it were, to creep round it. 
But should a chink give the opportunity for the new tissue to be 
deposited without obstacle in its original direction, the root will 
find its. way into it. The fibril will grow, by the nourishment 
sent to it, and by its own absorption, until it can no longer in- 
crease without the separation of the walls which confine it ; and 
this is accomplished by the force with which it imbibes fluid, 
which causes it to distend itself with great violence* This dis- 
tention is of the same character as. that of a piece of dry wood 
when exposed to moisture. 

1 09. The foregoing explanations will apply to a case of no un- 
frequent occurrence, in which a tree growing on one side of a road 
sends out roots to a ditch or stream on the other, — these roots 
dipping deep beneath the hard bed of the road, and rising again 
on the farther side. It is evidently by the slight drainage or per- 
colation of water, which will take place along this line, that the 
roots follow the same course. A more remarkable case, how ever, 

* It is by this same kind of force, resulting from Capillary Attraction 
(see Physics, Vol. I.) that mill-stones arc split in quarries. A long cylinder 
is first cut out; and grooves arc then chissellcd in its circumference, 
at the points where it is desired to divide it. Wedges of wood are (hen 
driven into the grooves, and these arc moistened ; by their violent expan- 
sion, the stone is split into the required number of parts. 

8* 



86 CURIOUS MODES OF GROWTH OF ROOTS. 

which has been more than once observed, is where the roots direct 
themselves along a naked rock, to reach water at a distance 
of perhaps twenty feet. It is not improbable that the constant 
ascent of vapour into the air in that direction, may be in part the 
cause of this curious mode of growth. An instance occurs in 
Leigh Woods near Bristol, which remarkably illustrates this ten- 
dency of roots to grow towards the spot most fitted to afford them 
nutriment. In a little hollow on the top of the shell of an old 
Oak, the outer layers of which, however, and the branches are 
still vegetating, the seed of a Wild-Service tree was accidentally 
sown. It grew there for some time, supported, as it would ap- 
pear, in the mould formed by the decay of the trunk on which it 
had sprouted ; but this being insufficient, it has sent down a large 
bundle of roots to the ground, within the shell of the Oak. These 
roots have now increased so much in size, that, as they do not 
subdivide until they nearly reach the ground, they look like so 
many small trunks. In the soil, however, towards which they 
directed themselves, there was a large stone, — about a foot square; 
and, had their direction remained unchanged, they would have 
grown down upon this. But about half a yard above the ground, 
they divide, part going to one side, and part to the other, and 
one of them branching into a fork, of which one leg accompanies 
one bundle, and one the other ; so that on reaching the ground, 
they enclose the stone between them, and penetrate on the two 
sides of it 

110. This example serves to show another fact, — that it is not 
in every case that we are to regard the parts of the axis which 
are above ground as Stems, and those which are beneath it as 
Roots. There is a tree peculiar to tropical climates, called the 
Pandanus or Screw Pine, in which the roots are always formed 
in somewhat of this manner. The stem is smallest at its lowest 
part, and it enlarges considerably above ; hence it would be veiy 
unsteady without some additional support ; and this is provided 
for by the transmission of the roots, not only from the bottom of 
the stem, but at different parts of its ascent. These grow down- 
wards in the air, and are provided at their extremities with a 
kind of cup which catches the rain and dew by which they are 



AERIAL ROOTS, DEVELOPMENT OF ROOT. 87 




Fig-. 27. Pandanus, or Screw Pine; a, 6, c, aerial roots 
partly serving as stems ; d, e, roots not yet reaching the 
ground. 

partly assisted in their elongation ; when, however, they have 
reached the ground, this falls off, and their extremities become 
true spongioles. When they begin to absorb nourishment from 
the earth, they increase greatly in diameter, and seem like so 
many assistant-stems. 

111. The general fact is, that the root is the portion of the 
axis which has a tendency to grow downwards towards moisture, 
and away from light, whilst the stem is the portion which tends to 
grow upwards, into the dry air, and towards light. This tendency 
is manifested during the earliest period of the growth of a plant 
from seed. Two parts always originate from it; one of which, 
termed the plumula (from its resemblance, when just unfolding 
to a little feather,) is the rudiment of the stem and leaves, whilst 
the other, called the radicle, is the young root. The first of these 
exhibits from the commencement a tendency to grow upwards, 



88 



DIRECTION OF GROWTH OF ROOTS. 




and the second a similar tendency to de- 
scend. The late ingenious experimenter, 
Mr. Knight, devised a means of showing 
that the direction of the roots is in p art 
owing to the force of gravity. He placed 
some germinating beans on the circum- 
ference of a vertical wheel, and made this 
revolve rapidly and continuously for several 
days. From the constantly varying position 
of the seed, the force of gravity was here 
neutralized; but a new force, was substi- 
tuted for it, — the centrifugal force, or 
tendency to fly from the centre, which a 
stone (for example) let loose from the cir- 
cumference of such a wheel whilst revolv- 
ing would exhibit.* The root, influenced 

by this force as ordinarily by that of gravity, 
Fig. 28. . 

Germinating seed, grew, in every instance, away from the 

a, plumuh; b t radicle, centre of the wheel, whilst the stem grew 
towards it. 

112. In another experiment, the wheel was made to revolve 
horizontally, so that the force of gravity continued to operate, 
but in combination with the centrifugal force ; in this instance, the 
direction of the radicle showed the evident influence of both; for 
it pointed downwards and outwards. Why the radicle should 
be thus influenced by the force of gravity, when the plumula 
grows in opposition to it, is a question that has not yet been an- 
swered ; but it is interesting to know the fact. We are as com- 
pletely ignorant of the ultimate causes of the most common oc- 
currences. We do not know, for example, why the earth should 
attract the bodies on its surface, or why it should itself be at- 
tracted by the sun. We only know the general fact that all 
masses of matter attract one another. 

113. Roots are ordinarily distinguished from stems, not only 
by their direction, but also by the presence of the absorbing 



* See Physics, vol. I. 



STRUCTURE AND DISTRIBUTION OF ROOTS. 89 

fibres, and by the absence of buds, which last are so characteristic 
of the stem, that any part on which they appear may be ordina- 
rily considered such, even though it is growing underground. In 
genera], however, the stem possesses the power of sending out 
root-fibres, as is shown by breaking off a slip of a branch and 
sticking it in the ground ; when, if properly attended to, it will 
usually form roots for itself, and soon become a new plant. But 
it is more rare to find the roots capable of forming leaves and 
flowers ; this, however, is the case in some instances, such as the 
Maple — a tree which may be completely inverted, the branches 
being buried in the ground, and the roots spread forth into the 
air, without being destroyed. 

114. The structure of the root-fibre, and of the spongiole which 
terminates it, may be beautifully seen in the common Duckweed, 
in which a single such fibre hangs from the under surface of every 
leaf, for whose nourishment it is destined. On looking at it with 
a magnifying glass, or microscope, a dark line is seen along its 
centre; this consists of the bundle of vessels by which that part 
is occupied. These are enclosed in a firm sheath of cellular tis- 
sue ; and at the point, this tissue is observed to be softer, more 
spongy, and less regularly formed. The extremities of the fibres 
are often seen to be covered with a little cap, which corresponds 
with the cup which the aerial roots of the Pandanus have been 
spoken of as possessing (§ 1 10.) 

115. The wide-spreading roots of a forest-tree do in reality 
consist but of bundles or collections of such fibres, strengthened 
by woody structure resembling that of the stem, and arranged in 
the same manner. The structure of the woody roots corresponds 
mainly with that of the stem, being Exogenous in Exogens, and 
Endogenous in Endogens (Chap. V. ;) but in the former no pith 
exists in the roots, their centre being occupied by vessels. The 
spread of the roots from the stem is usually greater than that of 
the branches ; so that the rain which is prevented by the latter 
from falling direct upon the ground, is directed just to that part 
through which the root-fibres are distributed, ready to suck it up. 

110. The force with which the roots absorb fluid is very 



90 ABSORPTION OF FLUID BY THE ROOTS. 

considerable. If a Vine be wounded in the stem when the sap is 
rising in the spring, a large quantity will flow out, and will con- 
tinue to do so for some time. An Elm tree, from which a por- 
tion of the bark and outer layers of wood had been accidentally 
torn off, has been known to pour forth from the wound many 
gallons in a few hours ; and the loss could only be restrained by 
nailing a leaden plate very firmly over the part. If the stem of a 
Vine be cut through, when the sap is ascending, and a piece of 
bladder be tied over the surface of the lower part, this is soon dis- 
tended with fluid, and, in a few hours will burst. Or if to this 
portion be fixed a bent tube, in such a manner that the fluid which 
rises shall have to lift a column of mercury, the absolute force 
may be measured, and this is found to be very great. 

117. The question, then, arises, — by what power do the roots 
thus absorb and force upwards through the stem the fluid of the 
surrounding soil 1 Is this power peculiar to the living system, 
or can we in any way account for it on the laws of mechanics 1 
Something analogous may be effected by the experimenter, with 
materials which he obtains from dead structures, or even from 
inorganic matter. If a glass vessel, of the shape delineated in the 
figure have its wide open mouth covered with a 
piece of bladder, and its cavity filled with rather 
thick sirup or gum- water, whilst the under side 
of the bladder is immersed in simple water, the 
volume of fluid within the glass will be much in- 
creased by the passage of water from without, 
through the bladder, to mix with the fluid in the 
interior ; and this entrance will continue as long 
ff * ' as there is much difference in density between 

the fluids on the two sides of the bladder. But there is also a 
contrary current to a less amount, — the interior fluid passing 
out to mix with the surrounding water. The increase in the 
volume of the fluid within the vessel is, therefore, equivalent to 
the difference in the amount of the opposite currents. If, on the 
other hand, the glass were filled with water, and immersed in 
sirup, it would be partly emptied by this action. 




ENDOSMOSE AND EXOSMOSE. 91 

118. The principal current is termed Endosmose (flow in- 
wards ;) and the lesser one is called Exosmose (flow outwards,) 
Bladder or animal membrane is by no means the only porous sub- 
stance which may be used for this purpose, though it is the most 
fitted to exhibit the experiment, from the rapidity of the action 
which takes place through it. Half-baked porcelain, and a pecu- 
liar porous limestone, have been shown to have the same pro- 
perties ; so that it is evident that this curious result is not depen- 
dent upon any modification of vital power. It is, in fact, but a 
peculiar form of capillary attraction.* 

119. Now the conditions requisite for this action are two fluids 
of different densities, separated by a septum or partition of a po- 
rous character. This we find in the roots. The fluid in their 
interior is rendered denser than the water around by an admix- 
ture (as will be hereafter explained) of the descending sap ; and 
the spongiole supplies the place of the partition. Thus, then, as 
long as this difference of density is maintained, the absorption of 
fluid may continue. But if the rise of the sap is due to the action 
of Endosmose, there ought also to be an Exosmose. This is 
found to take place ; for if a plant be grown with the roots in 
water, the fluid surrounding them is soon found to contain some 
of the peculiar substances they form, and which are contained in 
the descending sap ; thus, a Pea or Bean would disengage a 
gummy matter, — a Poppy would communicate to the water an 
opiate impregnation, — and a Spurge would give it an acrid taste. 

120. Thus we see how beautifully and how simply this action, 
extraordinary as it seems, is accounted for, when its whole history 
is known, on principles which operate in other departments of 
Nature. It has been asked, — why should not Endosmose take 
place when the roots of a dead plant are put in water 1 This 
question may be answered by another ; why does the wick of a 
lamp suck up oil only when it is lighted. The answer to both is, 
that it is only when the fluid already absorbed is in some way re- 
moved, that absorption can go on. In the latter case, as fast as 

* The attraction which causes fluids to rise into very minute tubes or 
pores. (Sec Physics, Vol. I.) 



92 ABSORPTION BY THE ROOTS. 

it is withdrawn by combustion, the lower part of the wick raises 
it by capillary attraction. In the former, as fast as the fluid is 
got rid of by exhalation from the leaves (Chap. VIII.,) the Endos- 
mose below will keep up the supply; but if the demand is sud- 
denly checked, (as when the plant is withdrawn from the influ- 
ence of light, or its vitality be destroyed by an electric shock,) 
there is no room for any additional fluid within the system, and 
the absorption is checked also. 

121. When the upper part of the stem is cut off, the sap will 
continue to rise by the force of Endosmose in the roots, so long 
as the fluid within is of greater density than the fluid without. 
But that will soon cease to be the case, the actions of the leaves 
being destroyed, and no descending sap being intermixed to keep 
up the^brce. These two causes, then, — the absence of any de- 
mand for sap in the leaves, and the cessation of the condition 
necessary for the maintenance of Endosmose,— are quite sufficient 
to account for its absence in the dead plant ; and its performance- 
soon becomes impossible for another reason, — the decay of the 
soft tissue of the spongiole through which it is performed. 

122. The nature of the fluid absorbed by the roots of plants 
will be more fully discussed in Chapter VI., when their food and 
the mode in which they are nourished by it will be described. It 
may, however, be stated here, that they appear to have a certain 
power of selection^ — some of the substances dissolved in the fluid 
which surrounds the roots being absorbed, and others being re- 
jected. Thus, if a grain of Wheat, and a Pea, be grown in the 
same soil, the former will obtain for itself all the silex or flinty 
matter which the water can dissolve ; and it is the deposition of 
this in the stem, which gives to all the Grasses so much firmness* 
On the other hand, the Pea will reject this, and will take up 
whatever calcareous substances (or those formed of lime and its 
compounds) the water of the soil contains, — these being rejected by 

* There is enough silex in a Wheat-straw to make a bead of glass 
when melted with potash with the blowpipe; and in the Bamboo it is 
sometimes collected in the knots in large masses, forming the substance 
called iabasheer in the East. 



ABSORPTION BY THE ROOTS. 93 

the Wheat. — Again, if the roots be placed in water coloured by 
any substance of which the particles are very minute, the finest 
of these will be absorbed with the fluid, and will be carried to the 
leaves ; whilst the coarser ones are left behind. In the same 
manner, if the roots be immersed in a solution of gum or sugar, 
a certain proportion only of these substances will be taken up 
with the fluid in which they are dissolved ; and that which re- 
mains will thus become gradually thicker. 



CHAPTER V. 

OF THE STRUCTURE AND FUNCTIONS OF THE STEM. 

123. The chief office of the stem appears to be, to elevate the 
leaves — which are organs destined to convert the crude fluid ab- 
sorbed by the roots into nutritious sap for the supply of food to 
the structure, — and the flowers,— in which this sap is applied to 
the production of new individuals, — into the most favourable po- 
sition for receiving the influence of light, heat, and air, on which 
their due actions depend. Accordingly we usually find it most 
developed in those kinds of plants in which one portion of the 
surface is set apart for the absorption of fluid, and another for its 
exposure to these influences. In the little plant which constitutes 
the Red Snow, and in others of a similar grade of organization, 
we find the whole surface adapted to absorb, and the whole sur- 
face equally exposed to air ; there is, therefore, no necessity for 
a stem. But as soon as, in the Mushroom tribe for example, the 
plant sends roots into the earth, it elevates the other portion above 
it by means of a stem ; and in this stem there is a set of channels 
or passages, which serve to convey the absorbed fluid from below 
upwards. But in these humble plants, destined to live but for a 
short time, and then speedily to decay, there is no necessity for 
providing the short stem with the toughness required in the trunk 
of the lofty forest-tree, which braves the storms of centuries. And 
accordingly we find that, whilst in the former the tissue is soft 
and cellular, resembling that of the rest of the structure, in the 
latter it is firm, and consists almost entirely of woody fibre, and 
this is consolidated by the deposition of hard matter within its 
tubes. 

124. Between these two extremes of softness and toughness, 



STRUCTURE OF THE STEM. 95 

there are a great many intermediate conditions. In Flowering- 
plants which only live for one year, and are hence known as an- 
nuals, the stem is usually herbaceous ; — that is, it consists almost 
entirely of soft cellular tissue, but contains, however, some bun- 
dles of woody fibre and vessels, which may be traced to the 
stalks of the leaves. These are commonly known as the strings 
of vegetables whose stalks or roots, are eaten, such as Asparagus 
or Turnips ; and that degree of stringiness which makes such 
plants unfit for the table when their growth is too far advanced, 
results from the increased formation of woody fibre which takes 
place towards the end of the season. In plants, however, the du- 
ration of whose life is greater, the stem gradually becomes consoli- 
dated by the formation in each year of a new set of these bundles, 
so that in time the soft cellular part bears but a small proportion 
to the whole. All stems, however, begin upon the same plan, 
whatever be their duration and ultimate density ; and the struc- 
ture of a first-year's branch of an Oak, for example, is essentially 
the same with that of the stem of an annual Pea. The foundation 
is laid, as it were, by an extension of the cellular tissue from which 
it springs, this being the kind of structure which most rapidly 
increases; and the consolidation of this is then gradually effected 
by the deposition of woody fibre in its substance. 

125. But the stems of Flowering-plants are not all formed 
upon the same plan ; in fact there are two different and nearly 
opposite modes in which the woody bundles are arranged in the 
stem ; and to one of these, they may be all referred. If the stem 
of the Asparagus be cut across, it will be seen that they are dis- 
tributed at intervals through its whole substance. On the other 
hand, in the stem of a Pea they would be seen regularly arranged 
in a circle, a little beneath the exterior. The former, if it con- 
tinued to live and grow, would have its new bundles deposited 
in the interior of the stem ; whilst the latter, if it survived a second 
summer, would have a new circle of woody bundles deposited on 
the exterior of the first. The former is then called an Enoogex, 
(growing from within ;) the latter an Exogen, (growing outside.) 
To the first division belongs the Palms, Bamboos, Canes, and 



96 



STEM OF EXOGENS. 



many other hard-stemmed and lofty tribes inhabiting tropical cli- 
mates ; but scarcely any except herbaceous plants belonging to 
this division exist in temperate regions ; the chief tribes which it 
includes in this country are the Lilies, and most other bulbous- 
rooted plants, and the Grasses. To the second belong all the 
trees and shrubs, and a large proportion of the plants of tempe- 
rate climates ; whilst between the tropics its place is partly occu- 
pied by the other. The last, as being the best known in this 
country, will be first described. 

1 26. If We cut across a young twig of any common tree or 
shrub, such as the Ash, Elder, &c. we shall observe that it con- 
sists of three distinct parts, ordinarily known as the Pith, Wood, 
and Bark. The pith is a soft spongy substance occupying the 
centre ; if a thin slice of it be cut, either across or vertically, and 
magnified, it is seen to consist entirely of cellular tissue, the cells 
of which are mostly of a very regular form. When young it 
contains a good deal of fluid, and has a greenish hue ; when the 
branch is older, it becomes white and dry ; and in an old stem or 
branch, it is often found to have shrivelled up and almost entirely 
disappeared. 

127. Now this pith is the first-formed portion of the stem, 
being in fact the remainder of that cellular structure, of which the 
whole was originally composed, but which gradually gives place 
to the woody portion ; and whilst at first it has to impart nou- 
rishment to the organs which are growing from it, this function is 
performed more perfectly by the vessels as soon as they are de- 
veloped in the stem, and the pith becomes of no farther use. The 
pith of a branch is always an extension of that of the branch or 
stem from which it sprang; and if the latter be cut through, just 
where a bud is rising from it, this will be seen to consist at first 
almost entirely of a kind of prolongation of the cellular portion 
which occupies its centre. The pith of trees is applied to no im- 
portant use. One curious product is obtained however from the 
large pith which constitutes nearly the entire stem of a herbaceous 
plant ; this is the substance known as Rice Paper, which is made 
by cutting the soft portion of the stem with a sharp knife in a 



STRUCTURE OF EXOGENOUS STEMS. 



97 




spiral manner, so as to cause it to spread out, as if a sheet of paper 
were being unrolled from a round ruler. It is then flattened out 
by pressure ; but its character, as cellular tissue like that of other 
piths, may be easily shown by magnifying a small portion of it. 

128. The pith is surrounded by the woody layers, the number 
and thickness of which depend upon the age of the branch or stem. 
In the herbaceous stem, the woody portion (as already stated) 

presents itself in the form of distinct 
strings^ arranged in a circle be- 
tween the large pith and the exter- 
nal skin or bark. Each string is 
separated from its neighbours by 
a prolongation of the pith, which 
thus maintains its connexion with 
the bark. In the stem of a second 
year, however, these bundles are 
found to form a second ring, en- 
Fig. 30. Diagram illustrating closing the first but still beneath 

the formation of the stem; a pith ; the bark wMch ig carr i e( l out . 

o, bark ; c, c, c, plates of cellular 

tissue connecting them, termed wards, as it were, to admit it. 

medullary rays ; d, d % d, woody rph e prolongations of the pith still 

bundles interposed between these. ■ . 

r exist ; but they become narrower 

in each ring, and at last appear merely as lines diverging 
from the centre. They are called medullary rays (or rays pass- 
ing off from the medulla or pith ; and their office is to maintain a 
constant connexion between the pith and interior of the stem and 
the bark or exterior, for an important purpose hereafter to be 
mentioned. The thin plates which they form, crossing as they do 
the direction of the fibres of the wood, are known as the silver 
grain by Carpenters. In many instances they add greatly to the 
beauty of the wood. (See Fig. 32.) 

129. The number of rings or layers of which the wood of any 
stem or branch consists, is in general easily reckoned by cutting 
it across ; and they correspond exactly in this climate with the 
number of years which the part has existed. There is reason to 
believe, however, that, — though in temperate climates, the trees 

9* 



9S 



STRUCTURE OF EXOGENOUS STEMS. 




of which shed their 
leaves and renew them 
once a year, a layer is 
formed no oftener, — in 
tropical climates, where 
many kinds of trees 
have two or three suc- 
cessions of leaves in a 
year, a corresponding 
number of layers will be 
formed. In this way we 
can account for the ex- 
IcHcAcde traordinary number of 
Fig. 31.. Horizontal or transverse, and per- layers in the Baobab 
pendicular section of the stem of an Exogen of trees of Senegal with- 
three years growth. In the centre of each is - . 

seen the pith, a, composed of cellular tissue ; out having recourse to 
surrounding it is the medullary sheath, b; and the supposition that 
exterior to it are three rings of wood, each con- ,, , mnn 

sisting of c, c, dotted ducts, and d, d, woody the ? Were aDOve 5000 
fibre.. The last-formed is in contact with the years old, as we must 
bark, e, e, in which the layers are indistinct. otherwise regard them> 

130. Each layer of wood consists partly of vessels or ducts, 
and partly of woody fibre. The former always lie next the centre, 
and are the parts earliest formed; the latter 
protects them on the outside, and is produced 
towards the end of the season. The mode in 
which these are arranged, however, varies in 
different trees; and it is principally this which 
gives that beautiful variety observed in sec- 
tions of many woods, when examined with the 

microscope. The ducts are at once distinguished 
a portion of an Ex- r ° 

ogenous stem in a by the large size of their orifices ; and the 

tharofThe'meduf woody fibres hy their com P arative minuteness, 
lary rays ; a, a, Amongst the ducts, however, we usually find 

wo"dv fibres- c o' lyhl = SOme elon g ated celIs ' which fiI1 U P the 
cut ends of the spaces between them, and which sometimes 
medullary rays. re semble woody fibres in form, but differ in the 




Fig. 32. 
Vertical section of 



HEART-WOOD AND SAP-WOOD. 99 

firmness of their walls. Whatever be the number of annual layers, 
they are always traversed by the medullary rays, which continue 
to extend outwards, with every addition to the diameter of the 
stem, even when the pith and the inner layers of wood have de- 
cayed away. 

131. In most trees, of which the wood is used as timber, the 
inner and older portion is much harder and dryer than the ex- 
terior. Sometimes there is an evident line of demarcation between 
the heart-wood, or duramen, as it called, and the sap-wood, or 
alburnum; this is seen, for example, in the lignum vitae, and coco 
wood, which are much employed by turners. But in most cases, 
the change of character is more gradual. This change is due 
to the consolidation of the interior wood, by the deposition in its 
tubes of resinous and other matter secreted by the plant. The 
portion of the stem in which this has taken place thus acquires 
great toughness and durability, but it is no longer fit to perform 
any office in the living system, save that of mechanically support- 
ing the rest; since no fluid can pass in any way through the 
now-filled-up channels. It is through the newer layers, or sap- 
wood, therefore, that the sap entirely ascends; and these, in their 
turn, become enclosed by others, and are at last consolidated, like 
the more aged ones, into duramen. The heart- wood alone is used 
by the artisan ; for the sap-wood soon splits and decays. 

132. As the pith and the inner layers thus gradually lose their 
original employment, and as in the outer part of the stem alone any 
active processes of vegetation go on, the former may be removed 
without injury to the latter ; and this is often naturally accom- 
plished by decay, which destroys the heart of an aged tree, with 
some portion of the exterior of the stem, but leaves the remainder 
a mere shell, still capable, however, of putting forth buds and 
branches, and of adding to its own thickness. 

133. The chief important variety in the structure of the Exo- 
genous stem is that exhibited in the Pine and Fir tribe. No 
ducts exist in their wood ; whilst the diameter of the tubes of the 
wood itself is greater than in other cases ; so that a horizontal 
section of the stem shows a series of openings of the same size. 



100 GROWTH OF EXOGENOUS STEMS. BARK. 

arranged with beautiful regularity, the division into annual layers 
being usually well-marked. As a general rule it may be stated 
that these are separated by the most distinct line in trees inhabit- 
ing temperate or cold climates, whose vegetating processes are 
entirely suspended by the cold after each layer is formed, whilst 
in trees of warmer regions, they pass into one another more gradu- 
ally. In the former, too, there is often a considerable difference 
in the thickness of the respective layers, according as the seasons 
have been favourable, or otherwise, to the formation of wood ; 
whilst in the latter their thickness is in general nearly uniform. 

134. These facts come to be of much interest, when we ex- 
amine the structure of the fossil plants, which are not unfrequently 
found imbedded in solid rock; for they thus afford evidence that 
the temperature of this quarter of the globe was both higher and 
more equable at the time they grew here, than it is at present; — 
an opinion which is equally supported by the nature of the fossil 
remains of Animals existing at the same period. 

135. Between the pith and the adjacent layer of wood, a de- 
licate membrane may be traced, which is termed the medullary 
sheath. This consists almost entirely of spiral vessels, which are 
seldom found in any other part of an Exogenous stem, except 
when they pass off from this towards the origins of the leaves. 
Their office is a very important one, as will be hereafter seen. 

136. The wood is enclosed by the bark, which is, like it, 
formed in regular layers, though these are much thinner, and can- 
not be so plainly distinguished. The layers of bark are formed 
from the interior, so that the oldest are on the outside. These are 
gradually lost, either by decay, or by falling off; so that it is very 
seldom that the same number can be traced in the bark as in the 
wood, although an additional one is formed in each at the same 
time. As the new layer of wood is formed on the outside of the 
previous one, — at the point, therefore, at which it is in contact with 
the bark — and as the new layer of bark is added to the inside of 
the previous one, — at the point, therefore, at which it was in con- 
tact with the wood, — it is obvious that they are produced at the 
same spot, and that the newest layers of both will always be i n 



FORMATION OF NEW LAYERS. BARK. 101 

contact with each other. Their production seems to take place 
in somewhat of this manner. At the end of the spring, the bark 
becomes loosened from the wood, with which it was previously in 
close contact, and a glutinous fluid, termed the cambium, is found 
between them. This may be observed by stripping the bark from 
almost any twig at that season. The cambium is gradually organ- 
ized into cells, and from these are formed the ducts and cellular 
portion of the woody layer, and the cellular portion (which is much 
the greatest) of the layer of bark. Later in the year, the woody 
tubes grow downwards from the leaves, obtaining nourishment 
from the fluid portion of the cambium as they descend, and at last 
partly uniting themselves with the vessels, &c. of the new woody 
layer, and in smaller proportion, with the tissue of the bark. 

1 37. In some kinds of trees, the bark contains a great deal of 
cellular tissue, and is therefore thick and spongy ; this is the case 
with those that furnish cork, which may be regarded as a sort of 
external pith. The inner layers, however, to which the name of 
liber is given, are usually thin and delicate in their texture, and 
have been applied to various useful purposes. One of these is in- 
dicated by the meaning of the term liber in Latin, which signifies 
a book; and thus in that language a book and the inner bark of a 
tree had the same name. The fact was that, before the invention 
of paper, the inner bark was one of the substances used by the 
Romans for the same purposes, as the leaves of the Papyrus 
(from which the term paper is derived) were employed in Egypt. 
It is the liber of other trees which is used by the islanders of the 
Polynesian Archipelago for cloth, mats, sails, &c. A very beauti- 
ful kind of liber is that obtained from the Vegetable-Lace tree (as 
it is called) of Jamaica ; when its layers are unfolded, it has the 
appearance of a delicate lace. 

138. From what has been stated as to the successive formation 
of new layers within, and the gradual loss of those on the exterior, 
it is evident that each layer of bark will in its turn be brought to 
the surface and be thrown off. On the other hand, each layer of 
wood is gradually being imbedded more deeply. Hence it follows 
that, if any substance be placed in the newest layer oi' wood, it 



1 02 AGE OF EXOGENOUS TREES. 

will gradually be covered by others ; so that if the tree be cut 
down at any future time, the number of years that have passed 
since it was imbedded may be known by counting the number of 
layers on its exterior. On the contrary, if the substance be not 
driven into the wood, but remains in the newest layer of bark, it 
will be gradually brought to the surface and will fall out. Such 
experiments have been tried for the purpose of showing the mode 
in which these two parts of the stem respectively grow. 

139. As the bark is sufficiently distensible to admit of the in- 
crease of diameter of the interior of the stem, there is no necessary 
limit to the age of Exogens ; and there are many unquestionable 
examples of such trees having attained an enormous longevity. 
In this country the Oak and the Yew appear to be the longest 
lived. At Ellerslie, the birthplace of Wallace, exists an Oak 
which is celebrated as having been a remarkable object in his 
time, and which can scarcely, therefore, be less than 700 years 
old. Near Staines, there is a Ye w tree older than Magna Charta ; 
and the Yews at Fountains Abbey, in Yorkshire, are probably 
more than 1200 years old. Eight Olive trees still exist in the 
Garden of Olives at Jerusalem, which are known to be at least 
800 years old. But the rate of increase in old trees is by no 
means the same as in young ; so that when they are grown for 
the profit to be derived from their timber, it is not advantageous 
to let them pass a certain age. Thus the rate of growth in the 
Oak diminishes greatly after about seventy years ; that of the 
Larch after sixty ; and that of the Elm after about sixty -five. 

140. It is in the vessels and woody tubes of the alburnum that 
the fluid absorbed by the roots is transmitted to the opposite ex- 
tremity of the stem ; and these vessels communicate with those of 
the leaves, which receive it from them. In the liber, on the other 
hand, the fluid which has been converted in the leaves into nu- 
tritious sap, descends again through the trunk, for the purpose of 
nourishing its different parts. Of this descending sap, a part is 
carried inwards by the medullary rays, which thus diffuse it 
through the whole stem, as also through the substance of the 
roots, down which it is conveyed by their bark. In this descent, it 



ASCENT OF THE SAP ENDOGENOUS STEMS. 



103 




mixes with the ascending current, especially at its lower part ; and 
being much superior in density, it adds to the density of that fluid, 
and thus maintains the conditions requisite 
for endosmose (§ 119.) The vessels down 
which the sap moves in the bark are of the 
branching character described as peculiar to 
those which convey the nutritious fluid 
(§ 87.) They form a complete network, in 
which the fluid may be seen to move in 
various directions. For this motion no defi- 
nite cause can be assigned. It does not 

Fig-. 33. depend on any impulse from above, corre- 

Branching vessels of _. , ,. , c ,, , ,. , 

the Bark. sponding to that action of the roots which 

raises sap in the stem ; for there is no power 
in the leaves to give any such force. It has been supposed to de- 
pend upon the gravity of the fluid, which will cause it to descend 
simply by its own weight ; but if that were the case, it would not 
ascend, as it often does, in the bark of the hanging branches of 
such trees as the Weeping Ash or Willow. It is only one, how- 
ever, of numerous cases in which a movement of nutritious fluid, 
through channels in the solid parts it supplies, takes place without 
any evident cause, in animals as well as vegetables. 

141. The stems of Endogens is formed upon a very different 
model. As already stated, the woody bundles in the stem of a 
year's growth, such as that of the Asparagus, are distributed 
through the whole of the cellular mass which originally consti- 
tuted it ; and a similar arrangement will be found in the stem of 
a Palm or other aged tree. Instead of being united into rings, 
these bundles remain separate; and it is only on the exterior of 
the tree, where they are closely pressed together in consequence 
of the continual addition of new woody matter to the interior, 
that they form any thing like hard wood ; and even this, though 
very useful for some purposes, does not possess the kind of texture 
which adapts it to the work of the artisan. Each annual set of 
woody bundles, which proceed (as in Exogens) from the leaves, 
passes, downwards in the softest part of the stem, which is its 



104 



STRUCTURE OF ENDOGENOUS STEMS. 



interior ; but after proceeding for some distance in this manner, 
it turns outwards, and interlaces itself with those which were pre- 
viously formed. In this manner, the lower part of the exterior 
of a Palm stem becomes extremely hard; partly from the pressure 
from within, to which it is not elastic enough to yield ; and partly 
from the constant interlacement of these new fibres, which wind 
themselves in among the dense tangled mass of the old, like roots 
seeking to pass through a stone wall. This density is sometimes 
so great, as to resist the blow of a sharp hatchet. 

142. The cellular portion of the stem, which in Exogens was 
separated, by the first introduction of wood, into pith and bark, 
here remains intermingled with the wood through the whole du- 
ration of life, as is shown 
in the accompanying 
figure. Each woody 
bundle contains ducts 
and spiral vessels, be- 
sides woody fibre ; and 
these are arranged in 
such a manner that the 
spiral vessels are on the 
side next the centre, and 
are protected by the 
woody fibre on the ex- 

»-** ag Wb T% VdTbZ - terior - The same ele " 

Fig. 34. Horizontal and vertical section of ments > therefore, exist in 
the stem of an Endogen, showing the bundles this Stem, as in that of 
of ducts, woodv fibre, and spiral vessels irre- ,, „ , , 

gularly disposed through the whole stem;a,a, the -^XOgen, but they 
portions of cellular tissne ; 6, 6, spiral vessels; differ in their mode of 
c, c, dotted ducts ; d, d, woody fibre. arrangement. From 

their peculiar structure, they increase very little in diameter, the 
hardness of the exterior not permitting their enlargement. The 
consequence of this is, that there is a limit to their age ; for the 
continual addition of new woody bundles to the interior so much 
compresses those which were previously contained in the stem, 
that they are no longer pervious to fluid, and the tree dies. Na- 




FUNCTIONS OF ENDOGENOUS STEMS. 105 

lure sometimes provides a remedy for this, in the splitting of the 
hard envelope, which allows the interior to dilate ; and this has 
been successfully imitated by art, — vigour having been restored 
to a Palm which had begun to languish, by splitting down the 
exterior of its stem with a hatchet. 

143. Although no complete distinction exists between the cel- 
lular and woody portions of the stem in Endogens,yet it is in the 
interior that the former predominates ; and hence some Palm- 
trees, as that which produces Sago, have been said to have a 
pith, which is (strictly speaking) erroneous. Through what chan- 
nels the ascending and descending sap of Endogens respectively 
move has not been ascertained ; but there can be little question 
that it is chiefly through the ducts that the former rises, whilst 
the latter finds its way downwards through the cellular inter- 
spaces. From the cellular portion of the stem, in Endogens as in 
Exogens, the buds take their origin ; whilst the roots are chiefly 
prolongations of the bundles of wood and vessels.. This is well 
seen in the PandanUs, where the bundles that ordinarily make 
their way downwards within the stem, and would there form part 
of the wood, pass outwards and become aerial roots (§ 1 1 0.) 

144. The same cause which sets a limit to the age of Endo- 
gens exempts them from the injurious effects which Exogens often 
experience from the compression of their stems by ligatures of 
various kinds. If a cord be tied tightly round the trunk of an 
Exogen, it will offer little impediment to the ascent of the sap; 
but it will obstruct its descent through the bark. In consequence, 
there will be a deficiency of nourishment to the parts beneath, and 
a superfluity above ; so that a protuberance will arise from the stem 
just at the point where the downward flow of the sap is checked. 
This protuberance will increase in progress of years, if the tree 
survive, so as almost to bury the cord beneath it ; but most com- 
monly the tree is destroyed ere long, by the insufficient supply 
of nourishment to the roots. 

145. Now such obstructions not unfrequently arise from na- 
tural causes. There are several creeping plants, whose habit it 
is closely to embrace the stems round which they coil ; such is the 

10 



106 GROWTH OF CREEPING STEMS. 

common Bindweed of this country ; and in tropical climates these 
creepers are more numerous, and their stems more woody. These 
seldom wind in complete rings, but in a spiral, growing like a 
corkscrew ; and thus the descent of the sap is rather obstructed 
than prevented. But an accumulation of the nutritious fluid takes 
place above the whole line of the spiral : so that, when the creeper 
is removed, the stem presents the curious appearance of a deep in- 
dentation passing round it from one end to the other, and on the 
upper edge of this a corresponding elevation. Endogens are sub- 
ject to no such alteration ; as the sap does not pass down the ex- 
terior of the stem, and its diameter increases but little. 

1 46. Such creepers are exceptions to the general rule that it is 
the tendency of stems to grow vertically or right upwards. This 
tendency is sometimes shown in a very curious manner. If the 
trunk of a young tree be artificially bent, by drawing it (for in- 
stance by a cord to one side, the branch which then most nearly 
approaches the vertical direction will increase more than the rest, 
and will at last appear quite continuous with the lower part of the 
stem. Again, if the trunk of a tree which usually throws out its 
branches almost horizontally, such as the Elm,- be broken off, the 
highest branches will gradually approach the upright position, so 
as to appear like continuations of the broken trunk. In coiling 
stems, however, it would appear as if some tendency to turn to 
one side was constantly operating in conjunction with the upright 
growth ; so that a cork-screw-like form is produced. 

147. It is a little remarkable that, though this turn is usually 
in the contrary direction to that in which the sun appears to move, 
(as is the case in the common Bindweed, most plants of the Pea 
tribe, the Passion-flower, the Dodder, and many others,) it is 
sometimes the same with it, as in the Hop. Almost all flowering 
plants, however, exhibit some tendency to a spiral growth in their 
stems. It will be hereafter shown that the regular arrangement of 
leaves on the stems and branches is in a spiral line. Moreover, in 
many smooth-trunked species, as the Cherry-tree, the bark is 
more easily torn off in a spiral than in any other direction. In 
trees having few branches, such as the Fir, it is not uncommon to 



HOLLOW STEMS. 107 

see the same tendency manifested by spiral fissures in the wood, 
when the bark has been for some time removed. The direction 
of this kind of twist seems to be as constant in straight stems, as 
in those which manifest it by coiling; thus, the Common Chestnut 
and the Horse-Chestnut have been observed always to twist in 
contrary ways. 

1 48. The stem is not always solid, either in Exogens or Endo- 
gens. Thus among the former, the well-known tribe of Umbelli- 
ferous plants presents many instances of a hollow stem, as in the 
common Hemlock ; and in the latter, the Grass tribe affords a 
corresponding example. In these instances, the hollo wness of the 
stem is due to the expansion of the outer portion faster than the 
interior can keep pace with it. The young stem is not hollow 
in either case ; and it is a beautiful instance of mechanical con- 
trivance, that, in these rapidly-growing plants, which are to be 
rendered independent of support from others, the limited quan- 
tity of hard tissue which they form should be disposed at such a 
distance from the centre, as to give the greatest strength with the 
least expenditure of material. If the material of a Wheat-straw, 
for example, were disposed in a solid form, it would make but a 
thin wiry stem, which would be snapped with extreme facility. 
In the hollow-stemmed Endogens, such as Grasses and Bamboos, 
and in many others, as the Sugar-cane and other Canes, we ob- 
serve certain divisions of the stem, which are called nodes or knots. 
Where the remainder of the stem is hollow, it is always solid here; 
and the partition has a peculiar degree of firmness, derived from 
the interlacing of fibres from all sides. And where the remainder 
of the stem is filled up (as in the Sugar-cane) with soft spongy 
tissue, there is still the same kind of firm division at the node. 
The space between the node is termed the internode; and from 
each one of these divisions, we usually find a single leaf-bud, or 
pair of leaf-buds, originating. The division into nodes is not so 
perceptible in Exogens; but it may be regarded as always existing. 
It is best seen in the young shoots of the Vine; where the fact 
that, from each internode but a single bud or pair oi' buds origi- 
nates, is equally evident. Hence, when the stem itself does not 



108 DIFFERENT FORMS OF STEMS. 

exhibit any distinct division by nodes, the Botanist is accustomed 
to regard them as existing near the points from which leaves or 
branches arise ; and to consider as internodes the spaces between 
these. 

149. Many parts are commonly regarded as roots, which are 
in reality stems. Their position, whether above ground, or be- 
neath the surface, is no criterion as to their real nature. It has 
been seen (§110) that roots sometimes grow in the air, and it is 
equally true that stems frequently grow in the earth. What are 
ordinarily called bulbous roots, for example, such as those of the 
Onion, Hyacinth, Lily, &c. are in reality underground stems. It 
was formerly stated (§ 1 1 1 — 113.) that the real distinctions be- 
tween the root and the stem consist in the contrary directions of 
their growth, and in the tendency of absorbing fibres to arise 
from the former, whilst the latter gives origin to leaf-buds. Now 
the base of the bulbls the real point of division between the stem 
and root ; for, whilst all below it, — namely the fibres which really 
constitute the roots, — has a tendency to grow downwards, the 
mass of the bulb, together with all above it, has a tendency to 
grow upwards. Farther, the scales of the bulb are in reality but 
leaves, changed from their usual character and aspect, or me/a- 
morpfiosed; and at the base of every one of these scales is found 
a little bud, occupying the same position in regard to it as the 
buds to the leaves on the higher parts of the stem. (§ 301.) Thus 
we perceive that here the stem is in a very contracted state, the 
internodes not being developed, and the leaves and buds of seve- 
ral nodes arising close together,. The difference between one of 
these scaly bulbs, therefore, and, the solid fleshy expansion of the 
root which constitutes a turnip or turnip-radish, is at once evident. 

150. But stems are sometimes so completely changed in their 
direction as well as form that they can scarcely be recognised as 
such, except by their tendency to, produce leaf-buds. Thus they 
sometimes creep along the ground, or even just beneath it, send- 
ing up buds, which develope themselves into branches, at inter- 
vals. Of this kind is the rhizoma or root-stock of most British 
Ferns, which creeps above ground in some species, and below in 



DIFFERENT FORMS OF STEM3. 



109 



others ; and the Ginger-plant of tropical countries has a stem of 
the sarfll character, which really furnishes Ginger, although this 
is commonly spoken of as the root of the plant, being partly 
buried beneath the ground. The runners of the Strawberry, again, 
are but trailing stems, which send down roots and develope buds 
at intervals, and thus extend the plant. This tendency, which is 
serviceable to man in this instance, is very troublesome to him in 
another; for in the same manner the Couch-grass overruns pas- 
ture lands, exterminating, if its increase be not checked, their 
original vegetation. As every internode of these trailing stems 
possesses the power of developing both roots and buds, it is use- 
less to attempt to destroy the plant by chopping the stem into 
pieces ; for this is in reality only multiplying it. It is well to 
mention, however, that, though usually regarded as a very rank 
weed, the underground shoots of this plant constitute a peculiarly 
nutritious food for cattle. 

151. One of the most distorted forms of the stem is that which 
presents itself in the Potatoe. This plant grows with an under- 
ground stem, sending up its flowering branches into the air, and 
sending its roots downwards into the earth ; but on this stem it 
forms at intervals the tubers or knobs, which constitute such an 
important article of food to man. That these tubers are still parts 
of the stem, is shown by their power of originating buds, from 
the points commonly known as the eyes of the Potatoe. When, 
therefore, we divide the tuber into pieces, keeping an eye in each, 
from every one of which we expect a young plant to spring, we 
follow in fact the same plan as that adopted in planting Sugar- 
Canes, which are not propagated from seed, but by dividing the 
stem into its internodes, and laying each of these separately in the 
ground. And thus it is seen that the division of the creeping 
stem of the Couch-grass effects in reality the same end. The 
quantity of fleshy matter deposited in the Potatoe serves for the 
nourishment of the growing buds before their roots are formed ; 
and thus it is that, if exposed to a warm and moist atmosphere, 
they are liable to sprout, without the contact of earth. It is re- 
markable that in their native climate (the tropical part of South- 

10* 



1 1 GROWTH OF ROOTS FROM BRANCHES. BANYAN TREES. 

America) the tubers of the Potatoe are extremely small, and that 
they become so when plants are raised from British stocks in any 
countries equally hot. 

152. In all these instances it is seen that not only buds but 
roots may arise from different parts of the stem and branches. 
But this tendency is by no means confined to such as grow on or 
beneath the ground. There are many trees of which the branches 
naturally hang downwards ; and if these reach the ground, they 
give origin to a new set of roots, which serve for their own nourish- 
ment, and for that of the shoots they send off, so that they become 
so many secondary stems. The most curious examples of this 
kind are the Banyan trees of the East Indies, of which one in- 
dividual sometimes constitutes a miniature forest. The most 
celebrated specimen is that of Cubbeer-bur, which many years 
since, possessed 350 principal trunks, and smaller stems amount- 
ing to more than 3000, every one of which was casting out new 
branches and hanging roots, to form future trunks. The space 
of ground which it covered was such, that it was estimated that 
7000 persons might have found ample room to repose beneath 
its shade. These trees are held by the Hindoos in superstitious 
reverence, and are dedicated to religious observances. Our own 
sacred poet, Milton, has given a beautiful delineation of it. 

"The fig-tree; not that kind for fruit renown'd; 
But such as at this day to Indians known 
In Malabar or Deccan, spreads her arms, 
Branching so broad and long, that in the ground 
The bending twigs take root, and daughters grow 
About the mother tree, a pillar'd shade, 
High overarch'd, with echoing walks between." 

153. The only Cryptogamia at present existing, which form 
true woody trunks, are the Tree-Ferns of tropical climates. In 
these, the stems which creep along or under the ground in the 
species inhabiting temperate climates, erect themselves into the 
air, and bear a beautiful crown of leaves. These stems are some- 
times hollow, and sometimes contain a sort of spongy pith. Their 




STEMS OF CRYPTOGAMIA. 1 1 1 

mode of growth is different from that of either 
Exogens or Endogens ; and appears of a sim- 
pler character. The stem, when cut across, is 
seen to consist of a number of hard woody 
plates adhering rather loosely together; and 
these, if traced upwards, are found to be either 
Fig. 35. continuations of the flattened footstalks of the 

TfaT^eJ-FernT leaveS which Cr0Wn the summit ' or to be the 
«, scars of former remains of those which have dropped off. Every 

year the leaves decay away, and are replaced 
by a new set formed above ; so that the stem continues increasing 
in length, but undergoes little change in diameter. The marks 
seen on the exterior of the trunk are the scars of the former leaves ; 
and by the relative position of these it is seen that, though the 
portion of the stem first formed increases but little in diameter, 
it receives some addition to its length, its scars being separated 
from each other by a much wider interval than in the newly- 
formed part. However, it is the general rule in these and other 
Cryptogamia, that the portions first produced undergo little sub- 
sequent change ; hence, whilst the names Exogens and Endo- 
gens are used to indicate the modes of growth respectively pecu- 
liar to the chief divisions of flowering plants, the flowerless plants 
may be included under the general term Acrogens, which inti- 
mates growth by the point, or by addition to the extremities 
only. 



CHAPTER VI. 

OF THE FOOD OF PLANTS, AND THE MANNER IN WHICH IT IS OBTAINED. 

154. A plant or tree can no more exist without food than can 
an animal ; and it is only because the mode in which they receive 
it is less evident to us, that we do not commonly think of vege- 
tables as equally dependent with animals upon the materials sup- 
plied to them by the elements around. We are constantly wit- 
nessing the act of feeding in all the animals that are under our 
notice; but the growth and reproduction of plants seem to take 
place with so slight an introduction of solid matter into their sys- 
tem, that it cannot be comprehended without farther examination 
how they derive the means of uprearing the gigantic masses of 
wood and foliage which many of them present to our admiring 
view. It cannot be shown that any solid matter is ordinarily 
taken up by the roots, except certain mineral ingredients which 
most plants require, and the use of which will be presently stated. 
How then, do they obtain the materials of the firm wood of their 
stems, roots, and branches, — of the soft but still firm tissue of their 
leaves and fruits, — -of the fleshy seeds they generate in their flow- 
ering system, — and of the various hard substances which they pro- 
duce in their different tissues 1 This question will now be an- 
swered. 

155. In the first place it may be laid down as a fact beyond 
doubt, that neither plants nor animals have the power of creating 
or producing matter which did not before exist. Living beings 
are entirely dependent upon the supplies they obtain from without, 
for the maintenance and enlargement of their own structures ; — 
they greatly alter the form and properties of the elements they 
take in ;■ — but they can create nothing. It is easy to say whence 



SOURCES OF THE FOOD OF LIVING BEINGS. lid 

every particle of which a living body consists is obtained by it ; 
for, by placing it in a variety of circumstances, and observing the 
changes in its mode of life which these produce, we can deter- 
mine the influence of each. Thus, an Animal may be fed exclu- 
sively on some one kind of aliment, as for instance sugar or gum ; 
and it is found that, however nutritious when combined with others 
such an article may be, it has not the power of supporting life for 
any length of time by itself, unless it contain (which no single ar- 
ticle of food except milk does) all the substances required by the 
animal for the right maintenance of its structure. So, also, on 
the food of Plants we may experiment, by placing them in different 
soils, and in different kinds of air, and supplying them with va- 
riable quantities of water; until we have discovered what is ab- 
solutely necessary to their growth, — what favours it, and what is 
superfluous or injurious. 

156. Before, however, we enter upon these inquiries, we 
shall derive much guidance from the knowledge of the substances 
actually contained at any given time in the vegetable structure. 
When we examine a seed, we find that it contains the germ of 
the new being; but that it principally consists (like the egg) of a 
nutritious substance prepared by its parent for the support and 
development of its offspring, until it is able to acquire food for 
itself; and it is by this means, as we shall hereafter see (Chap. 
XII.) that the young plant is enabled to push its first roots into 
the soil, and to elevate its first leaves into the air. By the time it 
has done this, however, all that store of aliment is exhausted ; and 
henceforth it is entirely dependent upon what it acquires for itself. 
The lowly plant developes itself in progress of years, by the won- 
derful power with which it is endowed, into the gigantic tree, in- 
creasing its weight from a few grains to many tons. Of what 
does its massive structure then consist 1 

157. The inorganic elements and mineral matter composing 
the solid earth on which we live, contain a certain number of 
substances which are termed simple, because they cannot, by any 
known chemical process, be shown to consist of others united 
together. Such, to take familiar examples, are the various metals, 



1 1 4 SIMPLE AND COMPOUND SUBSTANCES. CARBONIC ACID. 

with sulphur, as well as many other less common bodies. But 
the greater part of the substances which surround us are termed 
compound, because they can be separated into two or more simple 
or elementary bodies. Limestone, for example, when exposed to 
heat, is much changed in its character ; it gives off a kind of gas 
or air, termed carbonic acid gas ; and lime remains — a substance 
which is no longer safe to handle, on account of its possessing the 
power of destroying (or, as commonly said, burning) animal flesh, 
whence it is commonly termed quick-lime. But neither of these 
two substances are simple, for it is easily shown that carbonic acid 
is composed of two others, of which one is carbon, — a solid sub- 
stance of which the diamond is the purest form, but which is 
nearly the same with charcoal ; whilst the other is oxygen, a gas 
which forms part of the air we breathe. Again the lime may be 
shown (by a process of much difficulty) to consist of a metal, 
termed calcium, united with some of this same oxygen. 

1 58. Carbonic acid is the gas known as fixed air, — or in mines 
as choke damp. It is very injurious to the life of animals, acting 
as a sort of poison to them. It is formed during the combustion 
or burning of every substance containing carbon ; for this com- 
bustion consists of the union of carbon with oxygen, which, when 
it takes place rapidly, is accompanied by light and heat. Thus, if 
a pan of charcoal be burned in a closed room, a large proportion 
of the oxygen of the air will be converted into carbonic acid ; so 
that any human being, and almost any air-breathing animal, 
would rapidly lose his life in such an atmosphere. In this man- 
ner many persons have been suffocated. Charcoal, however, is 
not the only form in which carbon exists ; coal contains a very 
large proportion of the same element ; and carbonic acid is accord- 
ingly formed by its combustion. It also exists in the gas now so 
commonly used in towns for lighting, which is usually made from 
coal, though sometimes from oil; this gas consists of carbon in 
union with hydrogen, another kind of element presently to be no- 
ticed ; and when it is burned it forms carbonic acid, together with 
the vapour of water, which is produced by the union of the hydro- 
gen with the oxygen of the air. 



SOURCES OF CARBONIC ACID. 115 

1 59. Carbonic acid gas is also given off by all animals, which 
form it during the process of respiration or breathing ; for a por- 
tion of the oxygen which is taken into the lungs is there combined 
with carbon which the animal system wants to throw off, and then 
breathed out again in the form of carbonic acid. The presence 
of this gas in the air returned from the lungs is easily shown by 
breathing out through a tube, the end of which is immersed in a 
deep glass containing lime-water, which is water having a small 
quantity of quick-lime dissolved in it. After the lungs have been 
a few times emptied through this tube, the water becomes quite 
turbid, by the union of the carbonic acid with the lime, and the 
consequent formation of a carbonate of lime, which, not being so- 
luble in water, falls as a white powder resembling pounded chalk. 
It will be hereafter shown that vegetables, like animals, form car- 
bonic acid during the whole course of their lives, by causing the 
oxygen of the air surrounding them to combine with carbon which 
they have to give off (Chap. VIII.) 

160. Now the carbonic acid thus combined with the lime 
might be separated again by heat, or in other modes. If we pour 
a little vinegar upon limestone or chalk, a bubbling or efferves- 
cence is produced, which is caused by the acid of the vinegar com- 
bining with the lime, for which it has a greater attraction than has 
the carbonic acid ; and as they cannot both be combined with the 
lime, the latter is set free. It is set free, also, in large quantities, 
during the process of fermentation ; and this it is which renders it 
dangerous to walk over a vat in which fermentation is going on, 
and which extinguishes a candle held in such a situation. Putre- 
faction, too, is a kind of fermentation ; and carbonic acid is given 
off in this process. In fact, the respiration of plants and animals 
may be regarded as designed to carry off the carbonic acid pro- 
duced by a kind of slow putrefaction or decomposition, which is 
always going on within the body ; for it is a peculiar characteristic 
of the compound substances of which vegetable and animal struc- 
tures are made up, that they have a tendency to separate them- 
selves into their elements under the ordinary circumstances of 
warmth, moisture, &c. to which mineral bodies may be exposed 



116 TENDENCY TO DECAY IN ORGANIZED STRUCTURES. 

for centuries without change. This tendency to separation it is, 
which causes the decay of animal and vegetable substances after 
death ; for the elements that were previously combined in ways 
which no chemical pocesses can imitate, then pass off in simpler 
forms, of which carbonic acid is one of the chief 

161. Different parts of the animal and vegetable framework 
display this tendency in varying degrees. Thus the bones of an 
animal, and the heart- wood of a tree, may remain almost un- 
changed for centuries, and thus exhibit nearly the same perma- 
nence as the limestone rock; whilst the soft flesh of the animal, and 
the pulpy portions of the plant pass into decomposition almost im- 
mediately upon the death of the being. This decomposition, 
however, is chiefly remarkable after death, only because it is not 
then counteracted by the processes which form an essential part of 
the functions of life. The object of those functions is not only to 
provide for the growth of the structure, and for the production of 
new individuals which shall continue and extend the race ; but to 
maintain in constant perfection and vigour the parts already 
formed. This is accomplished by the removal of the portions which 
have exhibited the slightest tendency to decay, and by the depo- 
sition of freshly-formed substances of a similar character in their 
place. The particles which are removed are carried off in the 
blood of the animal or the sap of the plant ; and are separated from 
this in part by the process of respiration, which gets rid of the 
carbonic acid, and in part by other means of a corresponding na- 
ture. 

162. The rapidity of these processes of deposition and re- 
moval in the several parts of the living body bears a very close 
proportion with the natural tendency to decay which they re- 
spectively manifest. Thus, the bones of an animal are in general 
sparingly supplied with blood, and seem to undergo little change 
except as the result of disease or injury; but the supply of blood 
is greatly increased when any circumstances demand a new for- 
mation of this tissue. So, in the heart- wood of a plant, the circu- 
lation almost ceases; for so long as no air or moisture from with- 
out find access to the interior of the stem, this part retains its 



OBJECTS OF SUPPLY OF FOOD. 1 17 

firmness unchanged, and its particles require no renewal. But 
the soft tissues of an animal are largely supplied with blood, and 
also with absorbing vessels ; and the greatest part of the food 
taken in by one which has attained its full growth, is devoted to 
the maintenance of these parts in their right condition, which is 
essential for the proper performance of their functions. Of all 
these soft tissues, that which forms the brain and nerves is sup- 
plied with the largest proportion of blood, and undergoes the 
most rapid changes during life ; and it is this which most speedily 
decomposes after death. The soft tissues of plants do not so 
quickly decay as those of animals, and the circulation of nutri- 
tious sap through them is less active ; but still a movement of 
fluid takes place, and the same object is attained by it. 

1 63. Thus we perceive that food has for its object, — in the first 
place, to supply the materials of the growth and extension of the 
system, enabling the minute germ containing the seed to develope 
itself in time into a lofty tree ; — next, to maintain the parts so 
formed in their healthy state, by affording the materials by which 
those which have begun to decay may be replaced, — farther, to pro- 
vide a store capable of supplying the occasional extraordinary de- 
mand for reparation which disease or injury may produce ; — and, 
lastly, to enable the being to develope the germs of new individuals, 
and to supply them with a store of nutriment on which they may 
live until able to provide for themselves. Now of the 55 simple 
substances into which the solids, fluids, and gases, of the inorganic 
world may be separated, vegetables are principally made up of four; 
and of these only three exist in any large proportion. These three 
are carbon, oxygen, and hydrogen ; and the fourth is nitrogen. 

164. Of all these, Carbon is by far the most abundant. It is, as 
already mentioned, nearly identical with charcoal, which consists 
of the carbon of the wood, mixed up with a small quantity of earthy 
matter. If this charcoal be burned, it passes off in the form of 
carbonic acid gas, leaving a minute portion of white ash, which is 
principally of a mineral nature. It is chiefly to the carbon which 
it contains that the hardness and solidity of wood are due. In so 
large a proportion does it exist in that tissue, that when the other 

11 



118 SOURCES OF THE ELEMENTS OP VEGETABLE STRUCTURES. 

elementary bodies (the oxygen and hydrogen) have been separated, 
the carbon retains the form of the tissues in great beauty and per- 
fection, so that a section of a piece of charcoal will indicate the 
character of the wood from which it was made, nearly as well 
as would a section of an unburnt branch. On the other hand, 
in proportion as the tissues of the plant are deficient in carbon, 
do we find them deficient in firmness of structure. 

165. When we consider the large quantity of carbonic acid 
extricated by the respiration of animals, and by the immense 
amount of combustion of coal which is constantly going on in our 
large towns, there would seem no difficulty in understanding how 
it may be supplied to plants; but so vast is the extent of the at- 
mosphere through which the carbonic acid has to be diffused, that 
any given bulk of air only contains about 1-1 000th part of this 
gas. Hence it might be supposed impossible for the gigantic mass 
of carbon contained in the wood of a wide-spreading forest, to 
have been derived chiefly, if not entirely, from this source ; and 
yet such will be seen to be the case. For, although the soil may 
contain carbon, none of it is taken up in a solid form ; and its 
quantity rather increases than diminishes in the course of years. 

166. Oxygen is contained largely in plants ; and the presence 
of it in the air which surrounds them is very necessary to their 
healthful existence, — chiefly as affording the means by which, as 
already explained, the superfluous carbon is removed. This 
element is equally necessary to animals, and it constitutes about 
a fifth part of the air we breathe. A portion of this air is dis- 
solved, as it were, in water; and it is in this manner that fishes 
and other aquatic animals as well as plants are supplied with ox- 
ygen. Most if not all, however, of the oxygen which is contained 
in vegetable substances, is taken up by them either in combina- 
tion with carbon, or in union with hydrogen, — a body which with 
it forms water. 

167. Hydrogen is also contained largely in plants; and, in 
most of the substances into whose composition it enters, it is 
combined with oxygen nearly in the same proportion as in water. 
Although it is probable that a small quantity is introduced with 



SOURCES OF THE ELEMENTS OF VEGETAELE STRUCTURES. 1 19 

nitrogen in the form of ammonia (the pungent gas which gives 
strength to harts-horn, smelling salts, &c.) we may regard the 
water introduced into the substance of plants by their roots, and 
also in part absorbed by their general surface, as the chief source 
of this element, as well as of the oxygen contained in the vege^ 
table structure. 

168. Nitrogen has not been commonly regarded as an impor- 
tant element of the vegetable structure ; but it has been lately 
shown to exist largely in the growing parts of plants ; and there 
seems reason to believe its presence to be essential to the increase 
of their fabric by the formation of new parts. It is an important 
ingredient in the substance called gluten, which exists largely in 
the seeds of the various kinds of corn, and most of all in wheat ; 
and it is in part on this account that wheaten bread is the most 
nutritious of all vegetable substances ordinarily used as food, — 
since it approaches nearer in composition than almost any others 
to animal flesh, which contains a much larger proportion of nitro- 
gen than exists in most vegetable substances. It is, indeed, on 
account of their entire deficiency in nitrogen, that gum, sugar, and 
other similar products are not fit to maintain animal life by them- 
selves. Nitrogen constitutes four-fifths of the atmosphere ; but it 
does not seem to be taken in by the plant in its simple form. 
This gas with hydrogen forms ammonia, of which a minute quan- 
tity always exists in the atmosphere, being chiefly supplied to it 
by the decomposition of animal matter ; and this is absorbed by 
the soil and taken up by the roots, in the manner hereafter to 
be described. It is in the supply of ammonia which they yield, 
that the principal benefit of animal manures seems to consist. 

169. Besides these elementary substances, which all plants 
contain, and of which the vegetable tissue may be regarded as 
essentially consisting, almost all plants contain some mineral in- 
gredients, the presence of which is necessary to their healthy 
existence. These remain as ashes, when the other parts of the 
structure are set at liberty by combustion ; — the carbon uniting 
with the oxygen, and with some additional oxygen from the air, 
passes ofFas carbonic acid ; — of the hydrogen, part unites in the 



120 MINERAL SUBSTANCES CONTAINED IN PLANTS. 

same manner with oxygen, and passes off as watery vapour ;— 
the oxygen is thus entirely carried off, — and the nitrogen unites 
with the remainder of the hydrogen to form ammonia. Thus 
there remains nothing of the vegetable tissue but the incombusti- 
ble matter, and the nature of this varies in different plants. Thus, 
in the Grasses, (including Corn, the Bamboo, Sugar-cane, &c.) 
the ashes consist principally of minute particles of flint. In most 
other plants growing inland, we find some compound of the alkali 
potash ; and it is from this source that the greatest quantity of 
the pearl-ash that is largely used in various manufactures, such 
as soap and glass, is derived. On the other hand, in plants grow- 
ing near the sea, the potash is replaced by soda, which has nearly 
similar properties. Again, in most plants there is a small quan- 
tity of carbonate of lime, and in others there is a large quantity 
of lime combined with other acids ; thus, in Rhubarb we find 
large crystals of oxalate of lime ; and in the Corn-grains there is a 
considerable amount of phosphate of lime, by which their power 
of nourishing animals is greatly increased, since this substance 
constitutes the earth of bones. 

1 70. Of these different mineral ingredients, each plant seems to 
have some one or more that are as essential to its growth as in 
any other article of its food ; but the quantity required is some- 
times extremely minute, so as to be scarcely detectible, — -only a 
very small quantity of ash remaining after the tissue has been 
burned. In other instances, again, the mineral matter is so abun* 
dant as to present itself in the form of large crystals, which are 
deposited between the cells of the tissue. But that which seems 
its proper office is to form part of the membranous walls of every 
cell and tube of the whole structure, however delicate these may 
be. If a thin portion of almost any plant be burned in such a 
manner that free combustion of all its gaseous elements may 
take place, without disturbing the place of those which remain, a 
beautiful skeleton, consisting of extremely minute particles of mine- 
ral matter, will be seen, in which the form of all the cells, vessels, 
&c. may be distinctly traced. These particles would seem to be 
dispersed throughout the minutest parts of the vegetable tissue ; 



ASHES OF PLANTS. NOURISHMENT OP AERIAL PLANTS. 121 

and they probably serve the purpose of conferring additional 
strength upon the delicate framework of which it consists. Even 
in the finer ashes left by the combustion of common coal, a per- 
son to whom the forms of the elementary tissues of plants are 
familiar will often succeed in detecting with the microscope frag- 
ments of such skeletons, which thus add to the evidence — other- 
wise sufficiently strong, of the vegetable nature of that substance. 

171. Now that we are acquainted, therefore, with the elements 
of which the vegetable structure is composed, and have some 
knowledge of the sources whence these are derived, we are pre- 
pared to. inquire more minutely in what manner they are seve- 
rally received into the organism and made parts of its structure. 
This is an inquiry of the highest consequence in Agriculture — an 
art which, as it has been justly observed, is superior in importance 
to every other, since on it man entirely depends for his subsis- 
tence, and in great part also for the wealth and power obtained 
by commerce, and for the materials of his various manufactures. 

172i From what has been stated, it would appear that water, 
carbonic acid, and a minute quantity of ammonia, supply the in- 
gredients of the new compounds which are formed in the living 
plant ; but that, in most cases, mineral substances of some kind 
are required in addition. There are some plants which derive a 
sufficient quantity of all these elements from the atmosphere alone, 
to be able to maintain life, and even to flourish, without any 
other kind of supply. The water is absorbed by the general sur- 
face, but especially by the roots, which in such plants are usually 
long and of soft tissue throughout ; the carbonic acid is taken in 
through the green parts from the atmosphere alone, in the man- 
ner which will be described in the next chapter ; and the minute 
quantity of ammonia also contained in the atmosphere, which is 
probably dissolved in water and taken up with it, affords a suffi- 
cient supply of nitrogen. Such aerial plants usually contain but 
a very small quantity of mineral matters ; and these, too, are pro- 
bably derived from the atmosphere, in which, as will be hereafter 
mentioned, their particles are suspended. 

173. These aerial plants, clustering round the branches of 
11* 



122 GROWTH OF PLANTS IN AIR. 

lofty trees, and hanging to a great depth beneath them, are ex- 
tremely common in tropical climates, in which the atmospheric 
moisture is much greater, and where they constitute an important 
part of the vegetation ; and they are not wanting in this country. 
Many trees and plants which do not ordinarily grow in this man- 
ner may be caused to do so by accident or design ; and may even 
thrive extremely well. At New Abbey in Galloway shire, in the 
year 1817, there was growing on the top of a stone wall which 
measured ten feet in height, a plane tree, which measured twenty 
feet in height ; and, as it soon exhausted the bare and scanty soil 
in which the young plant grew, it sent down roots which clung 
to the side of the wall, and threw out neither bud nor branch 
until they reached the ground, which was not until several years 
had elapsed, during all which time the tree must have lived upon 
the materials supplied by the atmosphere alone. 

174. In one of the hot-houses in the Botanic Garden of Edin- 
burgh, a plant of the Ficus Australis (the Fig of New Holland) 
was caused to grow entirely without earth, by gradually with- 
drawing from the pots the several roots contained in them. The 
plant was well watered twice a-day, and put out roots freely from 
all parts of the stem and branches, by which it appeared to gain 
an ample supply of nourishment, for it produced a very full crop 
of fruit in the autumn after the earth was removed from the last 
set of roots. Even when a plant attaches itself by roots to the 
soil or rock, these may serve only for its support, and may not 
contribute any thing to its growth. 

175. Many succulent plants of warm climates exist in this 
manner; clinging to the faces of the barest cliffs, or rising out of 
the most dry and barren sand ; deriving their supplies of moisture 
and other aliment entirely, therefore, from the atmosphere. It is 
interesting to remark that most of these plants contain in their 
juices the substance caoutchouc (commonly known as Indian- 
rubber) and also wax ; and the moisture obtained from the atmo- 
sphere is prevented from evaporating (which even the thick cuticle 
would not prevent it from doing under the influence of a burning 
sun) by a thin layer formed by the drying of these juices around 



NATURE OF SOILS. 123 

them, which, like a waterproof cloak, keeps in the vapour that 
would otherwise be raised, so that the tissue of these plants be- 
comes turgid with their juices, although so little is absorbed. 

176. But the majority of vegetables require a larger and more 
certain supply of their various kinds of aliment than the atmo- 
sphere can furnish ; and, by the prolongation of their roots into 
the soil, they are enabled to obtain this, — in a manner, however, 
which requires some little explanation. What is commonly termed 
soil or mould consists of two kinds of ingredients ; — it is partly 
composed of the materials of the rock beneath, the particles of 
which are gradually separated from each other by the action of 
the atmosphere, of water, and of the roots of growing plants, as 
formerly explained Q 108 ;) — and partly of the remains of former 
races of plants, which are in process of decay. The former some- 
times exist almost alone ; and the latter, in land which has long 
been cultivated, often constitute a very large proportion. The 
two together, or either singly, will form a soil, the first use of 
which is to afford to the plant the power of affixing itself; so as to 
raise its stem, leaves, and flowers, into the most direct influence of 
the air and light. The next object which it should fulfil is to sup- 
ply the roots with a sufficient and regular amount of water ; and 
this will be effected according as it is capable of imbibing water 
readily from the atmosphere and from the neighbouring springs, 
ponds, or streams. 

177. Soils may be divided into the clayey, the calcareous 
(those containing much carbonate of lime) and the sandy. A stiff 
soil opposes the ramification of the roots, whilst a sandy one does 
not afford them sufficient hold ; it follows, therefore, that no plants 
will grow advantageously in the former but those whose roots do 
not naturally extend far, and whose vegetation is slow ; whilst 
those are most suited to a sandy soil whose roots spread exten- 
sively. It is by means of the Arundo arenaria or Sea-reed, that 
the Dutch attempt to check the progress of the drifting sand-hills 
which threaten desolation to large tracts of country ; and when the 
soil is once fixed by it, and improved by the decay of the indivi- 
duals first produced, it affords support to vegetation of more value. 

178. Again, every one knows that a stiff clay will retain its 



124 INFLUENCE OF SOILS. 

moisture for a long time, and that it parts with it or receives 
more with much difficulty ; whilst, on the other hand, a sandy 
soil absorbs much water, but soon loses it again. In climates 
where rain occurs pretty often, a calcareous soil is usually pre- 
ferred, as retaining its moisture sufficiently long, and yielding it 
with facility ; if, however, the temperature of a country be high, 
and rain fall but rarely, a stiff soil is to be preferred, as it will not 
become dry in the intervals ; whilst, on the other hand, a sandy 
soil answers better in a region where showers frequently descend. 
The defects of one soil in regard to its power of supplying vege- 
tation with moisture may be in some degree remedied by admix- 
ture with another ; this process is called in agriculture the temper- 
ing of soils. Thus, a stiff clayey soil may be tempered by mix- 
ing with it chalk, ashes, or sand, by which it is. rendered more 
permeable to water ; whilst, on the other hand, a loose sandy soil 
may be advantageously tempered with clay. 

179. The supply of moisture to the roots, however, is not the 
only important object which the soil should answer. It ought to 
afford carbonic acid also ; since it is essential to the rapid growth 
of a plant that this part of its nourishment should be taken in by 
its roots as well as by its leaves. The carbonic acid may be fur- 
nished in two ways ; either the soil may absorb it from the atmo- 
sphere, or the decay of some of the matter contained in it may 
disengage this product. It is a remarkable property possessed by 
several porous substances, of absorbing gases, and especially car- 
bonic acid gas, to the amount of many times their own bulk. Of 
all these, charcoal is one of the most powerful in this respect ; and 
it has been found that many plants may be grown in powdered 
charcoal, if sufficiently supplied with water, more luxuriantly than 
in any other soil. The charcoal itself undergoes no change, but 
it absorbs carbonic acid gas from the air ; this is dissolved by the 
water which is taken up by the roots, and thus it is introduced 
into the system. In such cases the plant derives its solid matter 
as completely from the atmosphere alone, as if its roots were 
entirely exposed to it ; for not a particle of the charcoal is dis- 
solved, and it, therefore, affords no nutriment to the plants. 



CARBONIC ACID OBTAINED FROM THE ATMOSPHERE. 125 

180. It may be thought incredible that the enormous quantity 
of carbon which enters into the composition of a single tree, much 
more of an extensive forest, and much more still of the immense 
succession of such luxuriant forests as those which formed our beds 
of coal, — should ever have been contained in the atmosphere; since 
any given quantity of air contains only about one-thousandth of its 
weight of carbonic acid, and this gas is composed of only about 27 
parts of solid carbon in every 1 00. But it must be remembered that 
as the weight of the air pressing upon every square inch of the 
earth's surface is 1 5 lbs., that pressing upon a square foot will be 
2160 pounds; and as the surface of the earth can be almost ex- 
actly calculated, it may be shown that, in the whole of th© atmo- 
sphere surrounding it, at least three thousand million million 
pounds of solid carbon must be contained, — a quantity which 
amounts to more than the probable weight of all the plants, and 
all the beds of coal which exist upon the earth. The quantity of 
carbon existing in sea- water is proportionably greater. 

181. The readiness with which the atmosphere yields a large 
quantity of carbonic acid to any substance having a strong attrac- 
tion for it, is shown when the walls and ceiling of a room are 
white-washed, or coated with a thin layer of quick-lime. This 
coating becomes very speedily converted, by combination with the 
carbonic acid of the air, into carbonate of lime. It may be thus 
shown that the atmosphere is capable of yielding to a coating of 
lime, extended over a given surface, and renewed as fast as it is 
converted into carbonate, three times as great a quantity of car- 
bonic acid as that which is taken in by the leaves and roots of 
plants growing upon a similar surface during the same time. 

1 82. The constant maintenance of this ingredient in the atmo- 
sphere, so as to supply the enormous drain upon it which active 
vegetation induces, is owing to changes of an opposite character 
taking place as constantly. Every animal is incessantly engaged 
in converting the oxygen of the air into carbonic acid, by the 
process of respiration or breathing. Of the solid carbon taken in 
as food, which is all derived, either directly or indirectly, from 
vegetable matter (since every animal is supported either upon 



126 FORMATION OF CAREONIC ACID IN THE ATMOSPHERE. 

vegetable substances, or upon the flesh of other animals which 
subsist on them,) a portion is constantly being restored to the 
gaseous form in this manner. A single man daily converts 45,000 
cubic inches of the oxygen of the air into carbonic acid by the 
carbon disengaged from his lungs ; and the enormous amount that 
must be daily formed by the whole human and animal population 
of the globe may thus be perceived. Again, the combustion of 
vegetable substances, — coal, wood, &c. — is a vast and continual 
source of the renewal of the supply drawn by vegetation from the 
atmosphere. It has been calculated that the small town of Gies- 
sen in Germany, possessing a population of about 7000 inhabi- 
tants, yearly converts more than 1000 million cubic feet of oxygen 
into carbonic acid, by the combustion of wood as fuel ; and in an 
English manufacturing town, where the proportion of coal used is 
far greater, the amount would be at least twice as much in pro- 
portion to the size. 

183. Now if it were not for the constant check which the 
processes of vegetation afford to the accumulation of this ingre- 
dient in the atmosphere, it would go on increasing, until the air 
became unfit for the support of animal life. But it is the fact, 
ascertained by the careful examination of the air preserved in 
some empty jars which had been buried with the city of Pompeii, 
that the proportion of the gases composing the atmosphere can be 
proved to have undergone no change during the last 1800 years. 
It is scarcely possible to contemplate all this wonderful system of 
mutual action, upon a scale so immense, without being struck 
with the simplicity and harmony of the design, and the perfection 
with which it operates. The plant is constantly withdrawing 
from the atmosphere its carbon, and converts it into the material 
of its own solid structures. Of the substances thus produced, a 
part is employed as food for animals and man, a part serves as 
fuel, a part is applied to various purposes in arts and manufac- 
tures, and a part decays without being removed from the place 
where it grew. Now nearly all the carbon taken in as food by 
animals is restored in a gaseous form to the atmosphere, either by 
the process of breathing during life, or by the decomposition of 



PRODUCTION OP CARBONIC ACID FROM VEGETABLE MOULD. 127 

their tissues after death; — all that is used as fuel is converted into 
carbonic acid gas, — as does nearly all that decays where it 
grew; — there only remains, therefore, the amount employed, 
chiefly in the form of timber, for various purposes by man, and 
this is more than supplied by the combustion of that which has 
been stored up ages ago for his use in the form of coal. 

184. It is from the decay of vegetable and animal matter that 
plants (at least under ordinary circumstances) derive whatever 
supply of carbonic acid they obtain in addition to that afforded 
by the atmosphere. Vegetable mould consists of decaying por- 
tions of the tissue of plants ; and is constantly liberating carbonic 
acid in the progress of its decomposition. This is dissolved by 
the fluid of the soil, and is taken up by the roots. The supply of 
carbonic acid thus obtained seems chiefly important to the plant 
when its leaves are undeveloped, as is the case in the early stages 
of its growth, as well as in every succeeding spring with all but 
evergreens. For it will hereafter be shown that it is almost en- 
tirely through their leaves that plants obtain carbon from the at- 
mosphere ; and when these are fully expanded, the absorption of 
carbonic acid by the roots may be dispensed with. 

1 8 5. But the decomposition of the vegetable matter of the soil 
requires the free access of air to every part of it. If any substance, 
however rapid its tendency to decay, be completely secluded from 
the atmosphere, little or no change in it will take place.* Every 
particle of the soil needs to be surrounded with oxygen, for the 
production from it of carbonic acid ; and to produce this condi- 
tion is one of the chief objects which is effected by tilling and 
loosening the soil. In this respect it is manifest that a clayey soil 
is inferior to all other kinds ; and its injurious character can only 
be remedied by admixture with other substances, or by laborious 
cultivation. The necessity of unimpeded access of air to the part 
of the ground through which the roots are distributed, is shown 
in an interesting manner when trees are planted too deep in the soil, 

* It is on this principle that various articles of food arc now preserved for 
subsequent use in tin cases completely closed ; and possess their perfect 
flavour after exposure to all varieties of temperature for several years. 



128 INCREASE OF CARBON IN SOILS. 

or when their roots have been covered with an additional quantity 
of earth. If the tree be old or sickly, it generally dies; but if it 
be vigorous, it sends out a new set of roots nearer the surface, 
and the extension of the old ones ceases. 

1 86. Notwithstanding, however, the gradual conversion of the 
carbon contained in vegetable mould into carbonic acid, and the 
absorption of this by the roots, the quantity of carbon in a soil 
which supports a flourishing vegetation is progressively increasing 
rather than diminishing. The addition takes place in several 
ways. The roots themselves throw out (as already stated § 119,) 
a considerable amount of matter formed in the vegetable itself, 
and corresponding in character with its peculiar secretions ; and 
this gradually undergoes decomposition, furnishing a large pro- 
portion of carbon. The leaves of plants which fall in the forest 
in autumn, and the old roots of grass in the meadow, are likewise 
converted into a rich vegetable mould, capable of yielding a large 
supply of carbonic acid; and thus it becomes evident that plants 
must absolutely derive more carbon from the atmosphere than 
they fix in their own tissues, since they are continually increasing 
the amount of vegetable mould on the surface of the earth. 

187. Thus we perceive that no matter which has been or- 
ganized can serve as the food of plants until it has undergone 
decomposition ; and that it is solely in the constant and regular 
supply of carbonic acid it affords, that vegetable mould is more 
adapted for the support of vegetable life than any other kind of 
soil. If we could form one of mineral substances only, in every 
portion of which carbonic acid should be slowly liberated, and 
which would be equally fit in other respects, it would equally 
contribute to the growth of the plants it supports. And thus we 
see a very important difference in the characters of the Animal 
and Vegetable Kingdom ; for, whilst the beings of the first group 
are entirely dependent for their nourishment upon matter that 
has been previously organized, and thus derive their support 
either from animal or vegetable bodies, — those of the latter are 
dependent for their growth only upon the materials supplied by 
the inorganic world, although their increase may be advantage- 



FOOD OF PLANTS AND ANIMALS COMPARED. 129 

ously assisted and stimulated by those which they derive from 
the decay of the former. 

1 88. And here again do we trace a beautiful harmony between 
the various parts of the grand scheme of Creation; for had 
vegetables been dependent, like animals, upon organic matter, 
both classes of beings must have gradually disappeared from the 
face of the earth, since the spontaneous death and decay of a large 
proportion of them is constantly restoring to the inorganic world 
the elements they have for a time held in those peculiar forms of 
combination which are termed organic ; and thus the amount of 
organic matter would be continually diminishing. But vegeta- 
bles, holding an intermediate station between the mineral and 
animal creation, bring them, as it were, into connexion with each 
other ; preparing, from little else than the air and the water of 
the globe, the materials for the sustenance of the countless mil- 
lions of beings which move upon its surface, and which, when 
their allotted period of existence has expired, restore by their de- 
cay the elements that are required for the support of vegetable life. 

189. No organic substances can be said to serve as food 
to vegetables in the same manner as to animals ; for they 
all need to be separated into nearly their simplest forms, before 
they can be reunited into the peculiar compounds which are 
required by the tissues of the plant for their nourishment and 
extension. If it were otherwise, we should expect that those 
would act as most serviceable manures which are most similar 
in composition to vegetable tissue ; just as animal flesh is the 
most easily digested of all food by the animal. But this is not 
the case ; for the richest manures are well known to be those 
which (supplying also certain ingredients presently to be men- 
tioned) are continually evolving by their decay a large quantity 
of carbonic acid. It is in part by hastening the separation of 
the elements of some substances which might otherwise resist 
decay for a long time, that lime acts as a valuable manure; and 
yeast is a still more powerful agent of the same kind, occasioning 
a kind of fermentation in the vegetable matter of the soil, by 
which a large quantity of carbonic acid is liberated. On the 

12 



130 



INFLUENCE OF MANURE. 



other hand, the carbonic acid produced by a manure may be too 
rapidly set free, and thus the plant becomes, as it were, gorged 
with food; whilst, at a subsequent time, it is starved by the 
deficiency occasioned by the too great energy of the change at 
its commencement. In such cases, the addition of some sub- 
stance (such as charcoal made from bones) which has the power 
of retarding decomposition, renders the operation of the manure 
more equable, and more correspondent with the progress of vege- 
tation. In general, rich manures are most serviceable to plants 
which, being only annual, naturally grow rapidly; and those 
which decompose slowly best suit a vegetation which increases 
with more regularity. 

1 90. These facts have an important influence on the operations 
of the cultivator, whether they be on the large scale of the farmer, 
or the small one of the gardener. No manure is more serviceable 
in yielding carbonic acid, than that which consists of decaying 
vegetable matter ; and this is much more abundant than is com- 
monly imagined. A small garden attached to a dwelling-house 
may be furnished with an ample supply of rich manure, by 
throwing into a pit all the refuse vegetable matter of the kitchen, 
and that supplied by the garden itself, in the form of weeds, dead 
leaves, prunings of fruit trees &c.; these should be lightly covered 
with earth, and kept slightly moist, and frequently exposed to the 
air by being turned over with the spade. And in a farm there 
will seldom be any deficiency of similar materials, if none are 
wasted. Weeds, for example, should not be burned, unless they 
are in seed ; for they may be made to afford a valuable supply of 
nutriment, instead of withdrawing it. A manure of this kind is 
to many plants more serviceable than that furnished by animals. 
Some remarkable examples are on record of the influence of it 
upon the growth of vines, which may be here advantageously 
introduced, as interesting illustrations of the foregoing principles. 

191. "Nothing more," says a vine-grower on the banks of 
the Rhine, "is necessary for the manure of a vineyard, than the 
branches which are cut from the vines themselves. My vine- 
yard has been manured in this way for eight years, without re- 



VINES MANURED WITH THEIR OWN CUTTINGS. 131 

ceiving any other kind of manure ; and yet more beautiful and 
richly-laden vines could scarcely be pointed out. I formerly fol- 
lowed the method usually practised in this district, and was 
obliged in consequence to purchase manure to a large amount. 
This is now entirely saved, and my land is in excellent condition. 
When I see the fatiguing labour used in the manuring of vine- 
yards — horses and men toiling up the mountains with unneces- 
sary materials, I feel inclined to say to all, Come to my vineyard 
and see how a bountiful Creator has provided that vines should 
manure themselves, like the trees in a forest, and even better 
than they ! The foliage falls from trees in a forest, only when 
the leaves are withered, and they lie for years before they decay ; 
but the branches are pruned from the vine about the end of July 
or the beginning of August, while still fresh and moist. If they 
are then cut into small pieces and mixed with the earth, they 
undergo putrefaction so completely, that, as I have learned from 
experience, at the end of four weeks not the smallest trace of 
them can be found." 

192. The following account from a poorer vine-grower, 
which is to a similar purpose, is instructive as showing of how 
much value a little intelligent observation may become. " For 
the last ten years I have been unable to place dung on my vine- 
yard, because I am poor, and can buy none. But I was very 
-unwilling to allow my vines to decay, as they are my only 
source of support in my old age; and I often walked very 
anxiously amongst them, without knowing what I should do. 
At last my necessities became greater, which made me more 
attentive; so I remarked that the grass was longer on some 
spots where the branches of the vine fell than on those on which 
there were none. So I thought upon the matter, and then said 
to myself: If these branches can make the grass large, strong, 
and green, they must also be able to make my plants grow better 
and become strong and green. I dug, therefore, my vineyard 
as deep as if I would put dung into it, and cut the branches 
into pieces, placing them in the holes, and covering them with 
earth. In a year I had the great satisfaction to see my barren 



132 



INFLUENCE OF MANURES. NITROGEN. 



vineyard become quite beautiful. This plan I continued every 
year, and now my vines grow splendidly, and remain the whole 
summer green, even in the greatest heat. All my neighbours 
wonder very much how my vineyard is so rich, and that I obtain 
so many grapes from it, and yet they all know that I have put no 
dung upon it for ten years." 

193. Although, therefore, it is probable that all plants and trees 
in full leaf could grow without any other source of carbonic acid 
than the atmosphere, an additional supply encourages that pro- 
ductiveness which it is the aim of the cultivator to obtain; and it 
is in the choice of his materials and the mode of their application 
that his skill and judgment are shown. The science of Vegetable 
Physiology has been but too little connected with the arts of the 
farmer and gardener ; and they have consequently been working 
in the dark, frequently coming, after tedious and unsuccessful 
trials, to conclusions which might have been drawn immediately 
from scientific principles. The certainty with which the mode 
of operation of manures upon vegetation has been now ascer- 
tained, should lead to most important improvements in practice, 
by which the productiveness of land may be much increased. 

194. Although carbonic acid and water are the chief sources 
of nourishment to plants, there is one element of great importance 
to their active growth, — namely nitrogen, — which is not contained 
in either of these compounds. It might be thought that, as so 
large a quantity of it exists in the atmosphere, no difficulty could 
exist in the introduction of as great an amount of it as might be 
desirable into the vegetable system. But it would seem that 
none of the elements of which that system is composed can be 
introduced into it in a simple form. Thus we have seen that the 
carbon is derived from carbonic acid, and the oxygen and hydro- 
gen from water ; and it is found that plants rather increase than 
diminish the quantity of nitrogen in the atmosphere. Nitrogen 
is introduced in the form of ammonia, the pungent gas which 
gives strength to hartshorn, smelling salts, &c. and which is libe- 
rated by the decomposition of almost all animal substances, in 
which nitrogen very largely exists. A great quantity of this gas 



SOURCE OF NITROGEN IN PLANTS. 1 33 

is thus being constantly set free and diffused through the atmo- 
sphere ; but still it forms so small a proportion of the whole, that 
it cannot be shown to exist in the air otherwise than in an indi- 
rect manner. Ammonia is very readily absorbed by water ; and 
thus the rain and dew, in descending through the atmosphere, 
become impregnated with it, although in very small amount. 
This ingredient can be proved to exist in rain-water ; and thus 
its presence in the atmosphere becomes certain. 

195. The quantity of ammonia which is thus supplied to 
plants appears sufficient for their ordinary growth. If, however, 
plants be set in powdered charcoal, sheltered from rain or dew, 
and watered with distilled water (which contains no ammonia) 
they do not flourish as they have been stated otherwise to do, 
but soon become stunted in their growth. This fact proves the 
great importance of the small amount of nitrogen thus introduced. 
There are many plants, however, to which a much greater supply 
of ammonia is necessary, on account of the large proportion of 
nitrogen which enters into some portions of their structure ; and 
such can only be cultivated to advantage when surrounded by 
additional sources of this material, such as are afforded by decay- 
ing animal matter of various kinds. For example, corn-grains 
include a large quantity of starch (which contains but little nitro- 
gen) with a certain amount of gluten, of which nitrogen forms a 
large proportion ; the latter is the most nutritious ingredient of 
the two, and it should be the object of the farmer to make the 
proportion of it as great as possible. This may be effectually 
accomplished by such animal manures as yield a large supply of 
ammonia. Thus, whilst corn grown in common vegetable 
mould contains about 66 parts of starch in every hundred, and 
only 9k of gluten, that which had been manured with blood or 
urine was found to contain 45 parts of starch, and 35 of gluten. 
It is by the use of a rich animal manure termed guano, that the 
barren soil on the coast of Peru is rendered fertile; this guano is 
collected from several islands in the South Sea, on the surface of 
which it forms a layer of several feet in thickness ; and it con- 
sists of the excrements of innumerable sea-fowl which resort 

12* 



1 34 ABSORPTION OF AMMONIA BY THE SOIL. 

there during the breeding season. It is sufficient to add a small 
quantity of guano to a soil which consists only of sand and clay, 
and previously contained not a particle of organic matter, in order 
to produce the richest crop of maize. 

196. Many kinds of soil have the power of absorbing ammo- 
nia like carbonic acid, from the atmosphere ; and thus add to the 
supplies which the plant obtains by its roots, so as to diminish 
the necessity for animal manure. Of this kind is gypsum, the 
utility of which has long been known, although the cause of its 
beneficial influence was not suspected. Gypsum powerfully 
attracts ammonia from the atmosphere, and yields it again to 
water which may soak through it ; so that as much ammonia as 
would supply the proportion of nitrogen to 100 lbs. of grass is 
yielded by little more than 4 lbs. of gypsum. The advantage of 
manuring fields with burned clay, and the fertility of soils con- 
taining iron, are to be referred to the same cause. Burned clay 
has, like gypsum, the power of fixing ammonia from the atmo- 
sphere, and of easily yielding it to water ; and minerals containing 
oxide of iron do the same, when separated into fine particles. 
Powdered charcoal possesses a similar action, and, indeed, sur- 
passes all other substances in its power of condensing ammonia 
within its pores, absorbing 90 times its volume of this gas. 
Decayed wood approaches very nearly to charcoal in this power ; 
and vegetable mould, which principally consists of wood in a 
more advanced state of decay, retains it in a very important 
degree, so that we perceive its influence on vegetation to be by 
no means confined to the supply of water and carbonic acid. 

197. On this account, vegetable mould is alone amply suffi- 
cient for the cultivation of all vegetables which contain but an 
average proportion of nitrogen ; but corn can only be grown to 
the greatest advantage, when the land is amply manured with 
those substances which contain the largest proportion of ammo- 
nia. This is yielded in the greatest abundance by the excrements, 
both fluid and solid, of animals, and particularly of man ; and in 
China, where, from the immense population, it is necessary to 
make the most of every foot of ground, the greatest care is taken 



SUPPLY OF MINERAL INGREDIENTS. 1 35 

to preserve these, and extraordinary fertility is the result. A 
similar practice prevails on some parts of the continent of Eu- 
rope ; and it is less successful, only because the mode of collect- 
ing the materials allows of the escape of a large proportion of 
the ammonia before the manure is used. 

198. By a judicious system of management, large towns may 
thus be rendered most important means of increasing the fertility 
of a country, and therefore of contributing to the supply of whole- 
some food ; instead of bringing together, as at present, so many 
causes of misery and unhealthfulness. 

199. The sources of what may be regarded as the essential 
ingredients of the food of plants having now been fully considered, 
(the more fully on account of the practical importance of the sub- 
ject,) we shall inquire into the influence of certain other materials 
which particular kinds require for their healthful growth, an in- 
creased supply of which tends greatly to their productiveness, and 
the influence of which ought, therefore, to be fully considered in 
the tempering of soils or the application of manures. These ma- 
terials consist of solid particles of various kinds, which are con- 
tained in the earthy portion of the soil, and which, being dissolved 
in the water, are taken up by the roots. Of these, some are im- 
bibed by almost all plants alike ; whilst others are retained only 
by particular kinds, so that they are either not taken up at all 
by plants of other kinds, or are secreted again into the soil, not 
being deposited in their tissues. 

200. There is considerable variety in this respect among the 
different tribes of plants ; each seeming to grow most advan- 
tageously when supplied with a certain kind of mineral matter, but 
being capable of taking up other forms in place of it, if it should 
be deficient. Thus the Rhododendron, like most other plants, 
deposites in its leaves and stem a large quantity of calcareous 
matter (lime combined with an acid, usually carbonic) when freely 
supplied with it. When grown in a calcareous soil, the ashes of 
its leaves have been found to contain 43] parts in 100 of carbonate 
of lime, and only f of silex or flinty matter; the ashes of the stem 
of the same plant contained 39 of calcareous earth, and £ of silex. 



136 SUPPLY OF MINERAL INGREDIENTS. 

But when grown in a soil in which silex predominated, the leaves 
of a similar plant contained 16f per cent of earthy matter, and 
2 parts of silex ; whilst the stem contained 29 parts of calcareous 
earth, and 19 of silex. 

201. It is curious to observe that, whilst calcareous matter 
seems principally deposited in the softer tissues, silex is found 
much more abundantly in the stem. This is especially the case in 
the Grasses, nearly all of which require for their healthy growth 
a large proportion of silex ; and this substance it is, which, being 
deposited in the slender tissue of the hollow stem, imparts to it a 
strength that seems disproportionate to the quantity of matter it 
contains. The silex may be melted by means of the blowpipe, 
into a bead of nearly the same appearance as glass ; and the fol- 
lowing curious instance shows the same effect upon a large scale- 
A melted mass of glassy substance was found on a meadow be- 
tween Mannheim and Heidelberg in Germany, after a thunder- 
storm. It was at first supposed to be a meteor ; but when chemically 
examined, it proved to consist of silex combined with potash, in 
the form in which it exists in grasses ; and, upon farther inquiry 
it was ascertained that a stack of hay had stood upon the spot, of 
which nothing remained but the ashes, the whole having been 
ignited by the lightning. 

202. Now the various substances which are thus required by 
plants for their healthy growth, are generally contained in the 
soil in sufficient amount to supply the majority of plants with the 
necessary material ; and some, when exhausted from the soil, may 
be supplied again from the water of the district, in which they 
are dissolved. It is partly by thus renewing what has been with- 
drawn, that the irrigation or flooding of meadows with water is 
very serviceable. Even where this water is of ordinary purity, 
containing scarcely any organic matter, and but very little mine- 
ral ingredients, the irrigation of meadows is very serviceable in 
improving their productiveness. From three to five perfect crops 
of grass have thus been obtained every year, by covering the 
fields with river-water, which is conducted over it in spring by 
numerous small canals ; so that the quantity produced in all was 



SUPPLY OF MINERAL INGREDIENTS. 137 

more than four times that which would have been obtained from 
one not so watered. In the neighbourhood of Edinburgh, the 
stream which conveys the fluids collected from the sewers of the 
town into the sea, has been diverted so as to cover much of the 
low meadow land which surrounds the town on three sides ; and 
the large quantity of organic as well as mineral matter which the 
fluid contains, is so beneficial to the growth of grass, that the pre- 
vious produce has been eight or ten times multiplied. This pro- 
ceeding is very injurious, however, to the health of the town ; 
since the offensiveness of the putrefying matter is very much in- 
creased by being diffused through a large quantity of water. 

203. The soil may be artificially supplied with the mineral 
substances required by different kinds of plants, as well as with 
those which yield carbonic acid or ammonia ; and the cultivator 
is frequently obliged to do this (though in general without under- 
standing the true benefit of the operation) especially when he has 
forced the growth in other ways. Thus if a meadow be manured 
only with gypsum (the use of which has been already mentioned,) 
the crops of grass will be at first greatly increased, but will after- 
wards diminish ; for the potash which the soil contained is soon 
exhausted by the rapid growth of the grass, and its farther in- 
crease is checked. But if the meadow be strewed from time to 
time with wood-ashes, which contain potash, the grass will thrive 
as luxuriantly as before. A harvest of grain may be obtained at 
long intervals on a sandy heath, by strewing it with the ashes of 
the heath-plants which grow on it, and which gradually collect 
the alkalies that are conveyed to them by water. 

204. It seems a remarkable fact that those plants of the grass 
tribe, the seeds of which furnish food for man, follow him like 
the domestic animals. The reason is that none of the corn plants 
can bear seeds that will yield a large quantity of flour, without a 
good supply of phosphate of magnesia and ammonia. Hence these 
plants grow only in a soil which contains these ingredients in ad- 
dition to the silex and potash already mentioned : and no soil is 
richer in them, than those where men and animals dwell together 
since these substances are largely contained in the animal body. 



138 SUPPLY OF MINERAL INGREDIENTS. 

and are set free in their excretions during life and by their gene- 
ral decay after death. Again, this fact explains why bone-earth 
is a most valuable manure to corn-fields ; since it consists almost 
entirely of these ingredients. The knowledge of it will also guide 
us in selecting the kind of wood of which the ashes will be most 
valuable ; for whilst those of the oak contain but a minute pro- 
portion of the phosphate, and those of the pine a quantity not 
exceeding the sixth part of their weight, those of the beech yield 
the fifth part; and thus with every 100 lbs. of the ashes of the 
beech, we supply a field with enough of these ingredients to serve 
for the growth of more than 15,000 lbs. of corn. 

205. It should be the object of the agriculturist, therefore, to 
ascertain the chemical character of the earthy portion of the soil 
which he cultivates, and to manure it with such substances as he 
finds will supply the deficiency for the particular plants which he 
wishes to grow. There are some soils which contain all the in- 
gredients required for almost an3* kind of vegetation ; and, when 
these cease to be productive, all that is necessary is to allow them 
to be fallow during a season or two. The atmosphere then acts 
upon the mineral particles, and causes that more complete separa- 
tion amongst them, which is necessary to prepare them for being 
dissolved in water and taken up by the roots of the plant. In this 
manner some soils prove extremely fertile, which scarcely contain 
a particle of vegetable mould, and have received very little animal 
manure. Thus the land in the neighbourhood of Mount Vesu- 
vius contains clayey earths, with chalk and sand, mixed in such a 
proportion as to give free access to air and moisture. This soil is 
produced by the slow decomposition and separation, through the 
action of the air, of the masses of lava which have at different times 
issued from the volcano, and which contain a great admixture of 
mineral matters, without a particle of vegetable mould. Now corn 
has been grown on this land for thousands of years with scarcely 
any manure; — the method adopted being simply this. A field is 
sown once every three years only ; and is in the intervals allowed 
to serve as a sparing pasture for cattle, which feed on the weeds 
that spontaneously spring up. But the influence of the weather 



EXHAUSTION OF SOILS. — FALLOWING. 139 

sets free an additional quantity of the mineral ingredients which 
the corn requires; the amount of nutriment contained in the seed 
is sufficient for the development of the young plants ; and the soil 
is of a kind extremely favourable to their subsequent growth. 

206. On the other hand, the very fertile land which was found 
by the first settlers in Virginia has been exhausted by a contrary 
proceeding. Harvests of wheat and tobacco were obtained for a 
century from one and the same field without the aid of manure ; 
but now whole districts are converted into unfruitful pasture-land, 
which, without manure, produces neither wheat nor tobacco. From 
every acre of this land there must have been removed in the space 
of one hundred years at least 1200 lbs. of alkalies in leaves, grain, 
and straw ; it became unfruitful, therefore, because it was deprived 
of every particle of alkali that had been reduced to a state capable 
of being dissolved; and it would require to lie entirely fallow for 
a great length of time to regain its fertility. Almost all the culti- 
vated ground in Europe is somewhat in the same condition. That 
of many of the West India islands has been also exhausted by the 
avarice of its former possessors, who have left it in a state which 
renders the cultivation of sugar much less profitable than formerly, 
since the Canes cannot be grown without a large quantity of ma- 
nure. If the ashes of the Canes, which, after the juice has been 
pressed from them, are burned to heat the pans for boiling down 
the fluid, were to be spread over the fields, the productiveness of 
the land would probably be much increased. 

207. It is not always necessary, however, that a field should 
lie fallow, in order to render it capable of producing some particu- 
lar kind of crop, the materials of which had been exhausted ; for, 
if it be sown with some vegetable of an entirely different kind, a 
profitable crop of this may be raised, whilst the land is renewing 
itself for the other. The power of doing this depends upon the 
nature of the ingredient which is deficient. It has been formerly 
shown that it is not the vegetable portion of the soil which is ex- 
hausted by the continued growth of any race of plants in the same 
spot, this being rather increased than diminished ; and therefore any 
plants which require no other nutriment than this maybe made to 



140 ROTATION OF CROPS. 

grow in a soil which wheat, or some other plant that takes up a 
large portion of some particular kind of mineral matter, had com- 
pletely exhausted. Such is the case, for example, with many of 
the Leguminous plants (the tribe including the Pea, Bean, and 
other similar vegetables,) which absorb so little mineral matter 
that they may be grown between two crops of corn, with nearly 
the same advantage for the latter, as if the land had lain fallow 
between. Hence these are called fallow crops. On the other 
hand, the injurious properties of many weeds that are apt to show 
themselves in corn-fields, result from their imbibing a large quan- 
tity of the same ingredients as those which the corn requires, so 
that, in proportion to the vigour of their growth, that of the corn 
must decrease. Hence it not only conduces to the neatness in 
the appearance of a corn-field, but also to its productiveness, to 
keep it free from weeds. 

208. Now the principle that a succession of different crops may 
be grown, where one could not be repeated without occasional in- 
tervals, has gradually superseded the old system of allowing the 
land to lie for a season, out of every three or four, entirely unpro- 
ductive ; so that the quantity of vegetable substances, nutritious 
either to man or beast, which is now raised from a given quantity 
of land is much greater than formerly. This principle has been 
fully established by experience ; but it is still acted on to a very 
limited degree, because its conditions are not yet fully understood. 
If there were nothing else to be considered than the kind of mine- 
ral substance which each plant draws from the soil, it would not 
be difficult to say what crops might succeed each other most ad- 
vantageously, since it would only be necessary to find out the 
mineral ingredients which each requires, to make those succeed 
each other which draw least of the same. But there is another 
very important condition to be attended to. 

209. Plants, as already stated (§ 1 19,) not only draw various 
substances from the soil, but impart to it a portion of the juices 
they have formed within themselves. A well-marked instance of 
this is the oak, which so completely impregnates the soil around 
its roots with tannin (the substance which gives to oak-bark its 



INFLUENCE OF CROPS UPON EACH OTHER. 141 

peculiar power of converting animal skin into leather) that few 
trees will grow in the spot from which it has been rooted up ; since 
this agent, even when a very minute quantity of it is dissolved in 
water, produces an effect like tanning upon the delicate tissue of 
the spongioles, and destroys their peculiar properties. It is pro- 
bable that every species of forest-tree produces a similar effect ; 
since it is well known that, when a wood composed of one kind 
has been cleared by the hatchet or by fire, the new growth 
which soon springs up, is not of the same but of a different spe- 
cies. Again, some of the plants which are known as the rankest 
weeds, secrete from their roots substances equally injurious to 
plants around them ; thus the Poppy tribe impregnates the soil 
around with a substance analogous to Opium, which is easily 
shown by experiments to have as injurious effect upon plants as 
an over-dose of this powerful medicine has upon animals ; and 
the Spurge tribe exudes an acrid resinous matter. 

2 1 0. The excretions of all plants seem injurious to themselves 
as well as to others of the same species grown in the same spot ; and 
in many instances, as in those just quoted, they are injurious to 
plants of other tribes also. But there are many instances in which 
they are absolutely beneficial to plants of distinct tribes. Thus 
most of the Leguminous tribe exude from their roots a matter 
analogous to gum, as may be easily shown by growing a pea or 
bean in water, which soon becomes turbid ; and this product is 
beneficial to plants of almost every other tribe. Hence, therefore, 
the benefit which the farmer derives from taking off a crop of 
beans between two crops of corn is not restricted to the value of 
the former ; since the succeeding crop of corn is absolutely im- 
proved by this proceeding. And, on the other hand, the ex- 
haustion of the soil by rank-growing weeds is not their only evil ; 
since they impart to it some of their own injurious properties. 

211. Now, by following out this system, and ascertaining 
what plants form the most abundant, and at the same time the 
most nutritious excretions, and what are the others which are most 
benefited by these, — keeping in view, also, the nature of the mi- 
neral ingredients they may respectively require, the agriculturist 

13 



142 PRINCIPLES OF AGRICULTURE. 

may hereafter be able to dispense almost entirely with artificial 
manure, as he has already done with the fallowing system ; — for 
he will only have to adapt his rotation to the particular soil, and 
then the excretions of one plant will serve as a manure to the 
other. Among the most useful for this purpose may be mentioned 
Lucern, which is remarkable for the extensive ramification of its 
roots, and the strong development of its leaves, and which requires 
but a small proportion of inorganic matter. This plant produces 
an abundant secretion from its roots, which, in the course of seve- 
ral years adds considerably to the quantity of vegetable matter 
in the soil, whilst its leaves serve as nutritious food for cattle. 

212. The subject of the foregoing Chapter has been treated 
in more detail than may seem consistent with the plan of this 
work ; because it is, of all departments of Vegetable Physiology, 
the one of most importance to the well-being of man. It can 
scarcely be doubted that, by improvements in the art of agricul- 
ture, the quantity of food for man and beast produced in this 
country, and the amount of those valuable articles which are cul- 
tivated in the various colonies of Great Britain, may be greatly 
increased. But these improvements cannot be carried into advan- 
tageous operation, until correct ideas on the subject have been 
generally diffused among those who are concerned in the work ; 
for, unless the principles on which they are founded are properly 
understood, it is more than probable that loss instead of gain will 
result from the attempt to introduce them. 

213. These principles may be thus recapitulated. 

I. The soil should be of such a character as to afford a steady 
supply of moisture to the roots, and to allow the air to penetrate 
it freely ; if it does not possess these requisites, it should be im- 
proved by tempering (§ 176 — 8.) 

II. In order to produce that luxuriant growth of plants which 
the agriculturist desires, the soil should afford a supply of carbonic 
acid to the roots, either by the decomposition of vegetable mould, 
or by absorbing the gas from the atmosphere. Its fertility may be 
increased by the addition of vegetable substances disposed to de- 
cay ; or by mixing it with charcoal, gypsum, or some other sub- 



PRINCIPLES OF'AGRICULTUKE. 143 

stance which possesses in a high degree the property of absorb- 
ing carbonic acid from the atmosphere (§ 179 — 193.) 

III. In order to effect the same object, the soil should be ca- 
pable of affording a supply of ammonia to the roots, either by the 
decomposition of animal matter contained in it, or by attracting 
that gas from the atmosphere. The proper supply of this important 
article of food may be secured, either by the employment of some 
animal manure which liberates it freely ; or by the admixture of 
some substance (such as gypsum or charcoal) which absorbs it 
rapidly from the atmosphere. The first method is most desirable 
when the vegetable substances which it is required to obtain in 
the greatest quantity, contain much ammonia, as do the seeds of 
corn ($ 194—8.) 

IV. In order to promote the luxuriant growth of any tribe of 
plants, the soil should be supplied with those mineral ingredients 
which its tissues naturally contain. If these are originally de- 
ficient, they must be added ; if they are contained in the soil, but 
have been for a time exhausted, the land should be allowed to lie 
fallow, until the action of the weather has farther separated the 
mineral particles (§ 199 — 206.) 

V. The soil may be improved nearly as much by a crop of 
a different kind, as by lying fallow ; provided that crop do not 
exhaust it of the same mineral ingredient with the one it replaces, 
and furnish excretions which are beneficial to it (§ 207 — 211.) 



CHAPTER VII. 



ON THE STRUCTURE OF LEAVES. 



214. The fluid which is so abundantly taken up by the roots 
of plants, and which is conveyed upwards along the interior of the 
stem, is very unfit for the nourishment of the structure, and for 
the supply of the growing parts, until it has been exposed to the 
influence of the air, by which great changes are effected in its 
properties. Now this object has to be attained in animals as well 
as in plants; and we observe two modes of effecting it. In some 
animals the blood is sent into very delicate external prolonga- 
tions of the skin, termed gills; through the thin membrane of 
which it may receive the required influence. And, although we 
usually see an apparatus of this kind existing only in animals 
which inhabit the water (the air diffused through which is really 
that which acts on the blood,) yet it is seen in some air-breathing 
animals also. The usual mode, however, in which the blood is 
exposed to the influence of the atmosphere in animals living on 
land, is by the introduction of air into cavities termed lungs within 
the body, constituting the process known as respiration or breath- 
ing; but this requires a series of movements for the constant ex- 
change of the air so introduced, in order that the portions rendered 
unfit for farther use by the changes that take place in it may be 
expelled, and a fresh supply admitted. A little consideration 
will show that to have introduced water in a similar manner, into 
the interior of the bodies of those animals which inhabit it, would 
require an immense amount of force, since water is so much less 
easily moved than air ; whilst, on the other hand, to have fur- 
nished air-breathing animals with external gills or other similar 
appendages, would have exposed them to great risk of injury 



GENERAL FUNCTIONS AND STRUCTURE OF LEAVES. 145 

would have impeded their rapid movements, and would have been 
attended with many inconveniences. 

215. Now in Vegetables, the same object is to be attained, 
but under different conditions. The nutritious fluid of the plant, 
like the blood of animals, needs to be exposed to the influence of 
the air to preserve its power of maintaining life ; and this cannot 
be effected either by the underground roots, nor by the hard 
woody stems and branches, which expose so small an amount of 
surface to the atmosphere. Nor can this be effected by the intro- 
duction of air into internal cavities in these parts ; since this would 
require a continued series of movements, as in air-breathing ani- 
mals, which the plant has no means of performing. Again, as the 
plant is rooted in the earth, and is not adapted to move through 
the atmosphere, there is no reason why its surface should not be 
spread out to any extent, for the purpose of exposing the sap to the 
influence of the air, just as the blood is exposed, in the gills of 
fishes and other aquatic animals to the small quantity of it con- 
tained in the water they inhabit. Farther, a very essentia] con- 
dition of the changes which the sap undergoes by coming into 
contact with air, is the influence of light, without which they 
would be very imperfectly performed. 

216. This general view of what is required from the leaves will 
suffice to show how beautifully their structure and situation are 
adapted to the offices they have to perform. The leaf may be said 
to consist essentially of an extension of the skin or cuticle of the 
plant into a flat expanded surface, which is supported by a skeleton 
prolonged from the wood of the stem or branch. If any leaf be 
but cursorily examined, it will be seen that from each surface a 
sort of skin may be torn, which may sometimes be stripped off 
very cleanly from the tissue beneath; — that the space between 
these surfaces is occupied by soft green tissue, which the naked 
eye can often perceive to consist of separate particles loosely 
united, and which is seen with the magnifying-glass to be com- 
posed of distinct cells, usually more closely-packed together near 
the upper surface than near the lower, whore there are many 
cavities or interspaces among them. 

18* 



146 GENERAL STRUCTURE OF LEAVES. LEAFLESS PLA>TS. 

217. The cuticle of the leaves is furnished more abundantly 
than those of any other parts, with stomata (j 91.) by which watery 
vapour and gases can pass out, and air can enter ; but these are 
chiefly, and often entirely, confined to the lower smface. The 
woody skeleton of the leaf forms what is commonly known as the 
midrib and the veins proceeding from it. These veins or nerves 
(as they are commonly termed) must not be confounded with si- 
milar parts in animals, since they do not in the least resemble them. 
They are principally composed of woody fibre and of spiral ves- 
sels and ducts : and they proceed from the neighbouring stem or 
branch, constituting the greater part of the footstalk or petiole of 
the leaf from which they afterwards spread out. 

218. This general account of the structure of the leaves of 
Flowering-plants will suffice to enable us to compare them with 
the corresponding parts in Cryptogamia. It also enables us to 
see how beautifully they are adapted, — by the immense amount 
of surface they present, — by its thinness and delicacy, — by its nu- 
merous apertures, — and by its expansion to the light of day, for 
the purpose they have to perform ; the exposure of the crude sap 
to the air and sun, under the influence of which it is elaborated 
or digested, so as to become a highly-nutritious fluid. 

219. There are few Flowering-plants in which the stem and 
branches are not, at some part of the year, clothed with these 
beautiful appendages ; and the exceptions are chiefly in those 
forms — unknown as natives of temperate climates, but common in 
tropical regions, — in which the stem itself is so altered in structure 
as to be able to perform the functions of leaves. Most of these 
are included under the designation of the Cactus or Prickly-Pear 
tribe. Their sterns, instead of being firm and woody, are compara- 
tively soft and fleshy ; their substance is moist, and composed al- 
most entirely of cellular tissue ; their surfaces are green, and co- 
vered with a distinct cuticle which is furnished with stomata; and 
their form is often flattened, so as to expose, like leaves, a large sur- 
face to the air. It is interesting to observe how completely the de- 
ficiency of one organ is here supplied by a modification of ano- 
ther. In almost all the plants of this tribe, there are tufts of prickles 



LEAFLESS FLOWERING PLANTS. 



147 




arising from regular points on the surface of the stem ; and to 
these the common designation of the tribe is owing. It will be here- 
after shown that these prickles are the rudiments of leaves ; and 
that under circumstances different from those in which the plant 
grows, minute leaves will arise from these very points. 

220. A conformation some- 
what similar may be observed in 
some plants of our own country ; 
thus in the common Butchers' 
Broom (Ruscus aculeatus,) the 
branches are flattened into a leaf- 
like form, and the flowers arise 
from the middle of their surface. 
In another foreign genus they are 
placed around the edges of similar 
organs. 

22 1 . There are, however, some 

_. __ ,„,.,, , „ flowering-plants of temperate cli- 

Fig. 36. «, leaf-like branches of 
Butchers' Broom, bearing flowers mates which are destitute not only 
in their centre ; 6, Xylophylla. G f leaves but of leaf-like surfaces. 
These grow by imbibing the juices of other more perfect plants ; 
just as parasitic animals obtain their food by sucking the blood 
of others. And as the juices which afford them support have 
already been elaborated or digested by the plant from which they 
draw them, they have no need of leaves or any similar apparatus 
for the purpose. Of this kind are the Orobanche, or broom -rape 
and the Cuscuta or dodder. Their branching roots are furnished 
with suckers by which they affix themselves on the bark of the 
plants round which they cling, and through which they imbibe 
their juices. 

222. Although there are few instances, then, in which leaves 
are absent in Flowering-plants, they are comparatively seldom 
found in Cryptogamia. In Ferns we always meet with them : 
and their general structure is much the same as that which will 
be described as characteristic of leaves in general. But, in addition 
to their other functions, the leaves of Ferns very commonly beat 



148 LEAF-LIKE ORGANS IN CRYPTOGAMIA. 

the fructification upon their under surface ; and hence the name 
frond has been given to them, for the purpose of distinguishing 
them from the leaves of the Phanerogamia, in which they never 
bear a part in the production of seedc In some of the Ferns, as 
the Osmundia regalis, or Flowering-Fern (as it has been incor- 
rectly called,) a handsome and well-known species common in 
England, the fructification is only borne on the edges of particu- 
lar leaves, which are much less expanded than the rest ; these 
are, therefore, called fertile fronds; whilst the other leaves, which 
here altogether resemble those of Flowering-plants in function, 
are called sterile fronds. In Mosses, we observe a large number 
of minute and delicate leaflets, having no concern in the fructifi- 
cation, which is entirely distinct ; but they have not those pecu- 
liarities of structure which distinguish the leaves of higher plants 
being destitute of a woody skeleton and of stomata ; and they 
seem to have a greater mixture of function, since they not un- 
frequently send out root-fibres, from their under surfaces for the 
purpose of absorption. 

223. We observe in the lower and simpler tribes, as has been 
heretofore shown, a much greater blending of different functions 
than in the higher, which possess a special organ for each, and in 
which they are consequently performed in a more energetic man- 
ner. Thus, when we descend below the mosses, we find no 
distinct leaves ; — they become, as it were, blended with the gene- 
ral surface; and all their functions are performed (as in the 
Cactus tribe) by this. Such, it will be remembered, is the case 
in the Liverworts (§ 32.;) and also in the Lichens and Algae, in 
all of which, however, we notice a flat expanded surface, by 
which the functions of leaves may be in some degree performed. 
This expanded surface is often of great extent in the Algae, and 
possesses a very leaf-like aspect ; but, as already stated, it does 
not perform the functions of leaves alone, but is every where 
equally adapted for absorbing the fluid that constitutes its nou- 
rishment, and in many instances contains the fructification also 
imbedded in its substance. (§ 41.) 

224. The case is different, however, in regard to the Fungi. 



DIFFERENT FORMS OF LEAVES. 1 49 

These plants derive their nourishment from matter which has 
already been in a state of organization ; and the condition in 
which they receive it is such, that it does not require to be elabo- 
rated by exposure to the atmosphere, as it does in all other but 
the parasitic plants already mentioned, which much resemble the 
Fungi in habit. Accordingly, the Fungi, the whole energy of 
whose vegetation seems to be concentrated upon the propaga- 
tion of the race, do not possess any thing analogous to leaves, and 
seldom exhibit even such an expanded surface as may be con- 
sidered to replace them. It is very rare, too, that this surface is 
green; and, as will be hereafter shown, this green colour in leaves 
is due to certain changes, which, from the condition of the growth 
just mentioned, the Fungi do not need to perform. 

225. We now return to the leaves of Flowering plants ; and 
shall trace in more detail their regular structure, the chief varie- 
ties of this, and the functions which they are destined to perform. 
And in the first place we shall consider their external aspect. 

226. The leaf is usually borne upon a petiole or foot-stalk; 
which connects it with the stem ; and it is at the bottom of the 
petiole that the separation from the stem takes place when the 
leaf falls off. By this it may be known what is really a single 
leaf, and what is a collection of separate leaves. Not unfrequently 
a leaf is very compound in its structure, consisting of a number of 
distinct leaflets, which might be regarded as so many leaves. 
But if these leaflets all proceed from one foot-stalk, and this drops 
off altogether at the accustomed period, they are to be considered 
as only the subordinate parts of a single leaf. Many such instances 
might be enumerated; but it will suffice at present to refer to 
the Ferns (§ 23,) in which what appears to be the stem is really 
but a leaf-stalk ; and what seem to be leaves are only leaflets 
proceeding from it, and forming part of one large leaf. 

227. There is not always such a definite distinction between 
the flat expanded blade and the round and slender petiole, as, 
from what we observe in common plants, we might suppose to be 
the case. The petiole is sometimes expanded into a leafy surface, 
and may even perform all the functions of the true leaves, when 




1 50 LEAFY SURFACE OF PETIOLES. 

the latter are deficient. Thus, in a British aquatic plant, known 
by the name of Arrow-head (technically Saggitaria sagittifolia) 
which is common in running streams, we observe what appear 
to be two kinds of leaves; some elevated above the surface, and 

formed like the head of an arrow, 
(whence the name of the plant ;) and 
others flattened, of equal breadth 
throughout, (technically termed ligu- 
late or strap-shaped,) and not ap- 
pearing above the water. These 
last are in reality the flattened pe- 
tioles, which perform the functions 
of leaves as long as they remain 
under water ; but as soon as any of 
them have strength to elevate their 

Fig. 37. Sagittaria Sagitti- summits above its surface, true 
folia, or Arrow-head; showing leaves are developed from Ihem, and 
expanded petioles beneath the .» ,. i ,, . , . , 

water, and true leaves above. the P etlole then contracts into a 
rounded form. 

228. A corresponding structure is exhibited by some of the 
Acacias of New Holland, which are sometimes so completely des- 
titute of true leaves as to be termed " leafless." When this is the 
case, the petioles are flattened and expanded, and present a leaf- 
like surface, which is adapted to perform the functions of the true 
leaves, from which they differ, however, in having the two sur- 
faces alike, and in growing in nearly an upright position, instead 
of horizontally. The true leaves, (which, like those of other Aca- 
cias, are very compound in their character, see Fig. 39,) are only 
to be found in young plants, or in old ones which have been freely 
pruned ; and it is not uncommon to find many degrees of deve- 
lopment, intermediate between those which exhibit the fully ex- 
panded leaf with its narrow and cylindrical petiole, and those 
which have no vestige of the blade, and present nothing but the 
leafy foot-stalk. In all these, the amount of expansion of the 
petiole bears a precisely inverse proportion to that of the leaf; the 
former replacing the latter when it is unfit to perform its func- 
tions. 



DISTRIBUTION OF THE VEINS. 151 

229. The blade of the leaf is composed of the expanded veins 
or nerves proceeding from the petiole, the interstices between 
which are filled up with cellular tissue, and the whole covered 
with cuticle. The mode in which these veins are distributed is 
very characteristic of the principal divisions of the vegetable king- 
dom. Thus, in the Cryptogamia, wherever true woody veins 
exist in the leaves (which is scarcely the case in any but the 
Ferns,) they are seen to divide and subdivide, — each usually bi- 
furcating, or splitting into two branches like the prongs of a fork, 
at intervals; — but these subdivisions do not unite again. Hence, 
as regards their leaves, the Cryptogamia may be characterized 
as forked-veined. 

230. In the group of Endogens it may be observed that the 
veins run in a nearly straight direction, and almost parallel to 
each other ; and that they have but little connexion by the inter- 
lacement of their minor subdivisions. The arrangement of these 
veins, however, differs according to the general form of the leaf. 
Thus, in the long narrow leaves of the grasses, and of other En- 
dogens, such as the Lily, Iris, &c, the foot-stalk is not continued 
along the leaf as a great central vein or midrib, but divides at 
once into several veins which run along side by side from one 
end of the leaf to the other ; and as there is very little connexion 
between these different veins, the leaf may be readily and very 
straightly torn from one end to the other. In other cases, how- 
ever, the leaf is broader, and the parallel veins are sent off from 
a large central midrib, running in the direction of the breadth of 
the leaf. As these, too, are but little connected with each other, 
it is easy to tear one of these expanded leaves into a number of 
narrow ribands, which will then hang from the midrib ; and some- 
thing resembling a compound leaf will thus be produced. As 
each of these ribands will have its own vein uniting it with the 
midrib, and is in its natural state scarcely connected with the 
surrounding parts except by the cuticle which envelopes the 
whole, the leaf will perform its functions nearly as well when thus 
subdivided as when entire. 

231. It is curious that such a separation should sometimes take 



152 DISTRIBUTION OF THE VEINS. 

place under the influence of natural causes. The Banana and 
Plantain of tropical climates have leaves of this kind ; and when 
they grow in situations in which they are much exposed to the 
wind, its action splits them up in this manner, from which they 
do not appear to suffer. These plants are sometimes grown in 
hot-houses in England ; and then, being completely sheltered, the 
expansion of their leaves is preserved entire, which seldom hap- 
pens in their native habitation. In whichever direction the veins 
are arranged, the general character of the leaf is the same : and 
hence the leaves of Endogens are spoken of as parallel-veined, 
by which character they are distinguished from those of the 
Cryptogamia on one hand, and, as will presently appear, from 
those of Exogens on the other. 

232. The form and mode of subdivision of the system of 
veins in Exogens are extremely irregular ; but there is a cha- 
racter common to all, by which the leaves of this group may be 
distinguished without much difficulty from those of the others. 
There is usually a midrib, or prolongation of the foot- stalk along 
the centre of the leaf, from which the smaller veins arise ; but 
sometimes the petiole subdivides at once into several subordinate 
veins, which run from one extremity of the leaf to the other, nearly 
parallel with the other, as in Endogens. But the secondary veins 
of Exogens, however they may be disposed, always give off a vast 
number of minute branches, which ramify and unite with each 
other, so as to form a complete net- work ; and thus it is, that the 
leaf of an Exogen can seldom be torn with any regular edge. 
From this character the Exogens may be described by their 
leaves as reticulated-veined; — the veins forming a reticulum or 
minute net-work. 

233. It is in these that we can make the most beautiful 
skeletons, by removing the soft fleshy portion of the leaves, and 
preserving only the woody structure. Such skeletons may often 
be found in the autumn, when the fallen leaves have been exposed 
to the influence of moisture for some time ; and with slight care 
they may be made to exhibit a very beautiful appearance. They 
may easily be prepared by soaking in water a leaf possessed of 



SIMPLER VARIETIES OF FORM. 



153 



firm texture, until its softer portion be in a state of decay ; if then 
the latter be washed away by carefully directing a small stream 
of water against it, the skeleton will be left. Not only do leaves 
contain such a skeleton ; but the leafy parts of flowers, and even 
the skins of such soft fruits as the cherry. 

234. Now, with the same distribution of the veins of the 
leaf, many curious varieties of structure may be produced by a 
difference in the degree in which the space between them is filled 
up. One of the simplest of these is where holes are left in the 
blade of the leaf, in consequence of a deficiency of the fleshy por- 
tion. Some plants are particularly liable to this irregularity; which 
does not exist, however, where they are well supplied with nou- 
rishment. A similar, but much more curious variety exists in an 
aquatic plant of Madagascar, in which the fleshy cellular tissue or 

parenchyma is so little de- 
veloped between the veins, 
that the living leaf much 
resembles in its form one 
of the skeletons just de- 
scribed. 

235. It is by no means 
uncommon to see the 
edges of leaves more or 
less deeply indented, ac- 
cording to the amount of 
nutriment which the plant 
is receiving ; the distribu- 
tion of the veins and the ge- 
neral outline of the leaf re- 
maining the same through- 
out. Thus the Cocklearia, 
or Horse-radish, has the 
edges of its leaves nearly 
Fig. 38. Different forms of leaves having smooth, when growing in 
the same venation; a, Water Crowfoot; a sufficiently rich soil ; but 
b, Jatroplia ; c, Passion flower; d, Sterculia 
platanifolia;/, Diohondra i g, Asarabaecu. » starved, the blade will 

14 




154 



DEFICIENCY OF FLESHY STRUCTURE. 



be divided into separate strips like the teeth of a comb, from the 
deficiency of flesh to fi]] up the spaces between the veins. In the 
accompanying figure are represented the outlines of different 
leaves having the same general distribution of the veins, but a 
different proportion of the fleshy substances between them. 

236. In some plants in which the framework of the leaves is 
very strong, the ends of this project from the edges of the leaf, 
when the latter is stunted in its development, forming sharp 
prickles. This is the case in the Holly ; the prickles on the leaves 
of which will be at once seen, if examined, to be simply the dried 
and projecting terminations of the veins. On looking at any full- 
grown Holly, considerable variety will be noticed in the degree 
in which the leaves have this prickly character ; and in general it 
is seen that the lower ones are the most stunted and rough, whilst 
the upper ones have the parenchyma of the leaf so much de- 
veloped as to include these extremities, and thus to render the 
edges of the leaf quite smooth. Sometimes it has been observed 
that a Holly, growing in a very luxuriant soil, has had all its leaves 
in this manner metamorphosed, so as entirely to lose the peculiar 
aspect of the tree. This is one of the modes in which the repul- 
sive character of some plants is softened down by cultivation. In 
the Cactus tribe, it would seem as if all the nourishment which 
would naturally form leaves is bestowed upon the stems them- 
selves ; and thus the tufts of prickles already noticed are the only 
indications of their place. These prickles are the woody veins ; 
which are sometimes seen, in specimens grown in hot-houses in 
this country, to be converted into true though very minute leaves ; 
in consequence, probably, of the greater supply of nourishment 
they receive under such circumstances, than in the dry and sterile 
situations they frequent in their native climes. 

237. The division of leaves into leaflets may be regarded as 
taking place upon the same general principle. When a single 
series of leaflets arises from the midrib, the leaf is said to be pin- 
nate, or winged. But sometimes, instead of leaflets arising from 
the midrib, we find secondary veins, from which, as from smaller 
midribs, secondary leaflets arise. Such are called bi~pinnate leaves. 



STRUCTURE OF COMPOUND LEAVE3. 



155 



The division may go yet farther ; and the secondary veins may 
give off their branches before any leafy parts appear on them, and 
these are of course much smaller and at the same time more nu- 
merous. A leaf in which this is the case is termed tri-pinnate. 
Such forms are generally peculiar to different kinds of plants ; but 
there are some in which we find them strangely intermixed, so as 
to display their real origin and character. 

238. Such an example is afforded by the Gleditsia or Honey- 
locust tree of North America, known to English gardeners by the 
name of the three-thorned Acacia. As in other Acacias, leaves 
are compound ; but the division often proceeds to such different 
degrees in different parts of the same leaf, that it is difficult to say 
whether it is to be considered pinnate, bi-pinnate, or (ri-pinnate. 

Of such a leaf, in the ac- 
companying figure, the 
central stem is the mid- 
rib ; and from it proceed 
the secondary veins on 
each side. The first pair 
of these bears on one side 
a leaflet which shows in- 
dentations marking a 
tendency to subdivision ; 
and, on the other side a 
series of secondary leaf- 
lets, formed by the com- 
plete subdivision of the 

first. The second pair of 
ok tribe, showing curious varieties in the mg b Qn Qne side 

subdivisions ot the leaflets. 

a series of secondary leaf- 
lets nearly similar ; but two of these are seen to have again sub- 
divided into more minute leaflets ; the distribution of the veins in 
which, however, precisely corresponds with that of the larger ones, 
so that a skeleton of the whole would exhibit little difference in 
its several parts. On the other side a portion of another series of 
secondary leaflets is seen; but towards the extremity they merge 
again into a larger leaflet. Below these again, we have a complete 




Fig. 39. Leaf of Gleditsia, one of the Aca- 



156 



SIZE OF LEAVES. BUDS FROM EDGES. 



pair of larger leaflets. If the whole of the leaf had been formed 
on this last plan, it would have been simply pinnate. If on the 
plan of the lowest division, in which there is a complete series of 
secondary leaflets on each side, the leaf would have been bi-pin- 
nate. And if the whole leaf had been constructed upon the plan 
of the minutely-subdivided portion of the second division, it would 
have been tri-pinnate. 

239. These are some of the most interesting varieties in the 
form of leaves, depending upon the degree in which the parenchyma 
or cellular flesh is supplied to fill up the interspaces between the 
veins. Of those which depend upon the various distribution of the 
Veins themselves it is not intended here to "speak ; since every plant 
furnishes materials for observation of these differences. In regard 
to the size of leaves, it may here be mentioned that, whilst in some 
species they are nearly microscopic, in others, especially of the 
Palm tribe, single leaves attain the length of from 30 to 40 feet. 

240. There are some leaves possessed of the power of deve- 
loping buds from their edges, — a fact which will hereafter be shown 
to be important. One of these is the Bog-Orchis (Malaxis palu- 
dosa) of English marshes ; in which these buds may be distinctly 
seen, though the whole plant is very small. A better example, 

however, is the Bryophyllum 
calycinum, which is a species 
inhabiting tropical climates, 
and known as the air-plant 
or leaf-plant, from the cir- 
cumstance of its having no 
true stem or roots, but main- 
taining its life, and even grow- 
ing and flowering, whilst 
hung up in a damp and 
warm atmosphere, without 

the contact of soil to any 
Fig. 40. Leaf of Bryophyllum calyei- rt of it The ]Mq budg 
num bearing buds at its edges. 

which develope themselves 

at the edges of the leaves, may become perfect plants before 




CHANNELS AND CAVITIES FOR FLUID. 



157 



separating themselves from the parent ; but, when they have once 
formed their own leaves and root-fibres, they are but little con- 
nected with it, and may be detached without injury. 

241. The usual form of leaves is often remarkably changed ; 
and many of the varieties produced in different plants seem to 
have for their object to collect water from the atmosphere and 
convey it to the roots. The large expanded leaves of the Arurn 
tribe, for example, have a deep channel down the midrib ; and 
this is continued along the petiole, so that the water collected by 
the leaf is conveyed to the point of the stem from which it 
springs. In the common Teazel (Dipsacus) of our own fields, 
and the Tillandsia, or wild Pine of South America, there are 
hollows capable of holding a considerable amount of water at the 
point of union of the leaf-stalk with the stem. 

242. But the most curious contrivances of this kind are those 
known as pitchers. The plants furnished with these curious recep- 
tacles are termed Pitcher-plants, and several kinds of them are 
known. In the Sarracenia, which is a native of Canada, these 

pitchers may be 
\\ distinctly seen to 
be formed by the 
very deep chan- 
neling of leaves 
and leaf-stalks, 
the edges of 
which fold to- 
wards and meet 
one another, so 
as to form a com- 
plete vase, the 
mouth of which 
is guarded by a 
sort of hood 

t?- *, n-,,- if j fiv. . •• i P farmed by the 

Fig, 41. Different kinds of Pitchers; a, pitcher ot 

Sarracenia; 6, pitcher of Nepenthes; c, pitcher of top o( the leaf 

CephaJotus. infoeNqienthes 

11* 




158 STRUCTURE AND USES OF PITCHERS. 

or Chinese Pitcher-plant, the pitcher is of more complex and sin- 
gular construction. The petiole, soon after it arises from the stem, 
spreads into a broad leafy expansion, which seems to perform the 
function of the true leaves ; it then contracts and forms a round 
tendril-like cord of several inches in length, and it then expands 
again, and is hollowed in its interior, so as to form a very capacious 
and elegant receptacle. The mouth of this is guarded by a sepa- 
rate little leafy cover, which is connected with it by a distinct 
joint ; and this is regarded by botanists as the true leaf. In one 
more variety, the pitchers of the Cephalotus utricularis, or mon- 
key-cups of South America (so named from its being reported that 
the monkeys quench their thirst with the fluid they contain,) the 
petiole seems to form the lid ; and the pitcher itself is composed 
of the hollowed leaf which hangs from it by a kind of hinge. 

243. In regard to the functions of these curious organs there 
is some difference of opinion. It seems probable that the pitcher 
of Sarracenia is a kind of fly trap, which serves to catch insects, 
the decay of which may furnish materials for its growth. Its in- 
terior is beset with long bristly hairs which point downwards ; and 
at the bottom there is poured forth from the plant a honey-like 
secretion, which is very attractive to insects. They experience 
little difficulty in reaching it ; but when they endeavour to return, 
they are checked by the downward projection of the hairs, and are 
caught like a rat in a trap. It has been observed that the plant 
does not thrive so well in a place from which small insects are ex- 
cluded ; and there is good reason to believe, therefore, (especially 
since, as we shall presently see, a corresponding instance certainly 
exists,) that they are in some way beneficial to its growth, proba- 
bly furnishing by their decomposition when dead a sort of 
manure which is useful to the plant. 

244. In regard to the Nepenthes no, very positive statement 
can be given ; and it is certain that, of the fluid which is found in 
the pitcher in the living plant, a part at least is poured into it 
from the plant itself, since it has been found to contain fluid while 
quite immature, before the first opening of the lid. The interior 
is covered with downy hair ; and it is probable that this performs 



PITCHER OF DISCHIDIA. 159 

the same functions as in other cases, attracting moisture from the 
atmosphere by its numerous points. It has been observed that 
the lid is closed in dry weather, as if to prevent loss of fluid by 
evaporation from the interior ; but that if the atmosphere be made 
very damp, and especially if the plants near it in the hot-house 
be watered, so as to cause a large quantity of watery vapour to 
surround it, the lid of the pitcher will open, and the quantity of 
water contained in it will soon show a considerable increase. 

245. The most curious, perhaps, of all the pitcher-plants at 
present known is one which has hitherto only been observed in 
India, growing in its native forests ; and it is called the Dischidia. 
It is a creeping plant, having a long twining stem which is desti- 
tute of leaves until near its summit ; and this may be a hundred 
or more feet from the roots, on which, therefore, it can scarcely 
depend for nourishment by absorption of fluid from the ground. 
Its supplies of moisture from a tropical atmosphere would be very 
uncertain if there were no provision for storing up what it occa- 
sionally collects ; but with such a one it is furnished. The pitcher 
seems formed of a leaf with its edges rolled towards each other and 
adherent; and the upper end or mouth, from which it is suspended, 
is quite open, and adapted to receive whatever moisture may de- 
scend from the air, whether in the form of rain or 
dew. It is accordingly always found to contain a 
considerable quantity of fluid, in which a number of 
small black ants are generally seen ; these are pro- 
bably attracted by it, and their decomposition may, 
as in the case of the Sarracenia, render it yet more 
nutritious to the plant. But the most curious part of 
the whole apparatus is a tuft of absorbent fibres, re- 
sembling those of the roots, which are prolonged 
_,. 40 n ' om me nearest part of the branch, or even from 
Pitcher or His- tno staJk to which the pitcher is attached, and which 
chidia Rafflesi- spread through the cavity. They may be regarded 
ania: showing 1 • ,, ,. , , c n ...... 

the tuft of mot- in the "£ n tof secondary roots, serving to introduce 

like libres pro- into the plant the fluid aliment collected in these 
from^he^ad' curioua reservoirs, which may be compared to the 
joining twig, stomachs o\' animals. 




160 



VENUS S FLY-TRAP. INTERNAL STRUCTURE OF LEAF. 



246. One more curious modification of the leaf may be no- 
ticed ; — that which forms the insect-catching trap of the Dionsea 
muscipula, a plant inhabiting the southern part of the United 
States, and commonly known as Venus' s Fly-trap. In this plant 
we find certain of the leaves fringed at their edges with a row of 
long spines, and endowed with the power of folding the two sides 
of the leaf towards each other, so as to enclose any thing between 
them which may have settled upon its surface. When thus 
folded, the spines cross each other in such a manner as com- 
pletely to prevent the escape of an insect which may be thus 
captured. Upon each half of the blade of the leaf, there are 
three projecting thorns ; and it is when either of these receives 
the slightest touch, that the two sides fold together and form a 
complete trap, the walls of which seem to press more closely 
upon the captive the more it struggles. Any unfortunate insect 
which alights upon the leaf is thus speedily destroyed ; and its 
decay appears to furnish the plant with nutriment beneficial to it. 
Plants of this kind, which have been kept in hot-houses in this 
country from which insects were carefully excluded, have been 
observed to languish ; but were restored by placing little bits of 
meat upon their traps, — the decay of these seeming to answer 
the same purpose. The petioles of the leaves which form these 
traps are very much widened and flattened, forming leaf-like 
organs, which seem to perform the functions of true leaves. 

247. Having now noticed the- chief varieties of leaves, as 
regards their external form, we shall proceed to consider their in- 
ternal structure ; and this exhibits a degree of complexity which 
would scarcely be supposed. The internal structure of the leaf 
cannot be well examined without a high magnifying power. It is 
necessary to cut the leaf across with a sharp knife, and then to pare 
off an excessively thin slice from the cut edge ; so that when a sec- 
tion, exhibiting the thickness of the leaf, from one surface to the 
other, is placed under the microscope, the light may be sent 
through it. A portion of such a section of the leaf of the Lily, 
wnich may be regarded as sufficiently characteristic of leaves in 
general, is shown in the next figure. The hollow cells of the cuticle 




INTERNAL STRUCTURE OP LEAVES. 161 

covering the upper 
surface of the leaf 
are seen above, and 
those of the under 
cuticle below. Be- 
tween these are 
seen a large num- 
Fig. 43. Section of the leaf of the Lily ; a, cells ber of shaded cells, 

composing cuticle of upper surface; b, cells of cuticle w hj c h represent 

of lower surface; c, c, stomata; d, upper closely-set 

layer of parenchyma ; e,f, lower rows of cells more the coloured paren- 

loosely arranged. chyma Qr fleghy 

portion of the leaf. These are arranged with considerable regu- 
larity, and are packed very closely together beneath the upper 
surface ; and there are scarcely any spaces between them in that 
part. Below, however, it is seen that the cells are of less regu- 
lar form, and that they do not come into nearly such close con- 
tact, so that there are many spaces amongst them, which, com- 
municating freely with each other, form what are termed the 
intercellular passages and spaces of the leaf. 

248. The stomata are chiefly to be found in the lower surface ; 
and they always open into the vacant spaces beneath the cuticle, 
and not against the cells in contact with it. It is the large pro- 
portion of these vacant spaces, which usually contain a consider- 
able quantity of air, that occasions the colour of the under side of 
the leaf to be usually much lighter than that of the upper. The 
cells of that part are themselves of a shade fully as deep ; but a 
much smaller number of them lie against the transparent colour- 
less cuticle. This may be easily seen by cutting a thin slice from 
the under side of the leaf with a sharp knife, in such a manner as 
to detach a portion of the cuticle with the cells adherent to it; and 
on magnifying this it will be observed, how small a proportion of it 
is really rendered green by the coloured tissue in contact with it. 
The large amount of air which these passages contain, is made evi- 
dent by putting a leaf in water under the receiver o( an air pump ; 
and if the pressure be removed from above, by exhausting the air, 
a number of minute bubbles will be seen to issue from the pores of 



162 PECULIAR ADAPTATIONS. OLEANDER. 

the leaf, which form a portion of the air previously contained 
in it. 

249. Nearly all the leaves which assume the ordinary posi- 
tion, having one surface directed upwards and the other down- 
wards, closely correspond with the one just adduced as an exam- 
ple in their general structure. There are, however, some curious 
exceptions. Not unfrequently we find the openings of many of 
the stomata blocked up; especially in plants which inhabit hot 
and dry situations, and in which, therefore, less superfluous mois- 
ture has to be exhaled through them. It is in such that we find 
the cuticle formed of more than a simple row of cells. The 
Oleander is a very remarkable example of this structure. The 
upper cuticle consists of three rows of cells filled with air alone. 
Beneath this we find two layers of green cells packed with ex- 
treme regularity and closeness, so as not to leave any passages 
between them ;. but between these and the lower cuticle the tex- 
ture of the parenchyma is very loose. The lower cuticle contains 
three and sometimes four layers of cells; and its exterior is covered 
with downy hairs. It contains few or no stomata ; but instead of 
these we find cavities in its substance, opening externally by a 
small orifice, and closely lined within by similar delicate hairs. 
NoWj as already mentioned, it seems that these hairs act as so 
many little rootlets ; absorbing moisture with which they may be 
in contact, when the necessities of the plant require it ; and no- 
thing can more effectually aid them than the little cavities just 
mentioned, which are so beautifully adapted to contain such mi- 
nute quantities of water as the atmosphere may deposite. Thus, 
the dryness of the soil and climate in which this species naturally 
exists is compensated by the peculiar structure of its leaves; and 
it is, accordingly, one of the few plants which will flourish in a 
sitting-room, the air of which is too dry for the health of most 
others. Similar cavities have been observed in the Nepenthes 
and Dionaea. 

250. There are many plants, however, whose leaves expose 
each side equally to the light ; their surfaces being upright instead 
of horizontal. In these, both sides are usually formed alike, and 



PECULIAR ADAPTATIONS. WATER-LILY. 163 

their colours are the same. Upon examining their interior struc- 
ture it is found that both sides are equally furnished with inter- 
cellular passages ; and that the number of stomata above these is 
nearly equal. This is the case, for example, in the common Iris. 
But there are some instances in which the general plan of struc- 
ture is completely reversed, — the stomata being restricted to the 
upper surface, and the upper part of the parenchyma being much 
looser in texture than the lower. This is the case, for example, 
with the Water-Lily, and other plants whose leaves float on the 
surface of the water. The thick spongy leaf of the Water-Lily 
contains a large amount of air-channels, which serve to give it 
buoyancy; but these are all immediately beneath the upper sur- 
face, and communicate with the external air through its numerous 
stomata ; whilst in contact with the lower surface, — which, as it 
lies upon the water, is cut off from the actions that are usually 
performed by it, — are two rows of closely-packed cells, corre- 
sponding to those generally in contact with the upper surface. 
In all these instances we observe such a beautiful adaptation of 
the structure of these wonderfully-organized beings to the cir- 
cumstances in which they are to live and grow, that the intelli- 
gent observer can scarcely feel a doubt of the Wisdom and 
Omnipotence of the Designing hand which contrived it. 



CHAPTER VIII. 



OF THE FUNCTIONS OF LEAVES. 



251. It is in the leaves, as already stated more than once, that 
those changes are effected, which convert the crude fluid absorbed 
by the roots (consisting as it does of little else than water in which 
is dissolved a very minute proportion of the various matters exist- 
ing in the surrounding soil) into the proper juice or nutritious 
sap, — capable not only of supplying to the different parts of the 
structure the materials necessary for the maintenance of their 
healthfullness, the repair of injuries, and the production of entirely 
new parts, — but also of furnishing the ingredients of those several 
products, which the various tribes of plants may be said almost to 
create from the elements around them, and which are so valuable 
to man as articles of diet, as medicines, or as articles of use in his 
various manufactures. Many of these will have to be considered 
hereafter under the head of Secretions; but it is interesting to ob- 
serve here, that, — although almost every tribe of plants forms some 
substance peculiar to itself, some of which are of a highly poison- 
ous character, whilst others are of the mildest and most whole- 
some nature, — they all originate in ascending sap, which is of 
nearly a uniform character in each tribe. 

252. In this process of elaboration, as this conversion has been 
termed, several distinct changes are involved. The first is the 
concentration of the fluid by the loss of a considerable proportion 
of its water, so that the amount of solid matter contained in any 
quantity of it is much greater than before. This is effected by a 
process which resembles the perspiration of animals ; a large 
quantity of watery vapour being given off, under favourable 



LOSS OF FLUID FROM THE LEAVES. 165 

circumstances, from the surface of the leaves as from the pores of 
the skin of a man. 

253. If a glass vessel be placed with its mouth downwards 
on the surface of a meadow or grass-plot during a sunny after- 
noon in summer, it will speedily be rendered dim in the interior 
by the watery vapour which will rise into it, and this will soon ac- 
cumulate to such a degree as to run down in drops. From an 
experiment of this kind repeated by Bishop Watson during seve- 
ral successive days, on a meadow which had been cut during a 
very intense heat of the sun, and after several weeks had been 
passed without rain it was calculated by him that an acre of grass 
land transpires in 24 hours not less than 6400 quarts of water. 
This is probably an exaggerated statement ; as the Bishop does 
not seem to have been aware how completely transpiration is 
checked during the night; but it will serve to give an idea of the 
enormous amount of fluid which must be thus disengaged. Any 
person walking in a meadow on which the sun is shining power- 
fully, especially in a hot day in summer, when the grass has not 
long previously been refreshed by rain, may observe a tremulous 
motion in distant objects, occasioned by the rising of the watery 
vapour ; exactly resembling that which takes place along the sea- 
shore, when the sun shines strongly on the pebbles that have 
been left in a moistened state by the retreating tide. 

254. It is necessary, however, to distinguish the evaporation 
which is the cause of the latter occurrence, from the peculiar func- 
tion we are now considering ; which as we shall see, is influenced 
by circumstances that only act during the life of the plant, in such a 
manner as to prove it to be something of a different character 
from that which we observe in dead substances. All moist bodies 
exposed to a tolerably warm and dry atmosphere have a tendency 
to become dry, — the fluid they contain slowly passing off in the 
form of vapour. The rapidity witli which this takes place will 
depend upon the amount of heat to which they are exposed, and 
the degree of dryness of the surrounding air. Every one knows 
that warmth is of great assistance in drying moistened substances 
of any kind; and this results from its promoting the conversion 

15 



166 CONDITIONS OF SIMPLE EVAPORATION. 

of the water into vapour. It may easily be observed, too, that a 
damp atmosphere retards the process ; and the air sometimes has 
so large a quantity of vapour suspended in it, that it deposits it 
as a dew upon dry substances, instead of raising fresh moisture 
from damp ones. 

255. Now the living fabrics of plants are subject, like all other 
moist substances, to the loss of fluid by evaporation ; and this 
would take place under the conditions just mentioned, from all 
the parts which have this character, were it not for the protection 
afforded by the cuticle. This membrane, as formerly stated, 
covers the whole surface of every plant which is exposed to the 
air ; and, from its dry nature, and the absence of any fluid in its 
cells, it is not liable to be thus influenced by heat or dryness of 
the atmosphere, so that it effectually protects from the undue in- 
fluence of these agents the soft tissues beneath. The difference 
which results from the presence or absence of this cuticle may be 
well seen by comparing the long-continued freshness of the leaf 
of any flowering plant which is kept in the dark (so that its exha- 
lation, or transpiration of fluid through the stomata, as presently 
to be explained, is prevented,) and the rapid shrivelling of the 
frond of a sea-weed, or of any Flowering-Plant that naturally 
grows beneath the water, when equally exposed to the influence 
of a warm and dry atmosphere. And, as already noticed, the 
cuticle is almost invariably found to be the thickest and firmest 
in plants which frequent very hot and dry situations. 

256. Nevertheless the cuticle does not entirely check evapo- 
ration ; but this takes place from the surface of a dead plant, or 
any portion of one, as well as from one in the most active vege- 
tation. The shrivelling of Apples long kept, and the loss of 
weight of Potatoes, are examples of this slow and gradual change. 
It may be stated, then, that Plants, like other moist soft sub- 
stances, are liable to part with some portion of their fluid by eva- 
poration, especially when exposed to a warm and dry atmosphere ; 
but that the amount of this loss is far too small to account for the 
large quantity of vapour, which, as just stated, may be easily as- 
certained to pass off at certain times from the surface of the living 
plant, 



PASSAGE OF VAPOUR THROUGH STOMATA. 167 

257. Now a few simple experiments will show that there is 
a strong probability that this rapid transpiration takes place 
through the stomata. If a piece of glass be held near the upper 
surface of the leaf of a vine actively growing in a hot-house, little 
effect will be produced upon it ; but if it be held near the under 
surface, the glass will soon be dimmed by the vapour, and in a 
short time longer this will accumulate so as to form drops. As 
the upper surface of a vine-leaf is nearly destitute of stomata, 
whilst the lower is thickly covered with them, the disproportion 
in these effects is at once explained, if the transpiration really take 
place through these apertures. Similar experiments on other 
plants lead to the same general result. Where the stomata are 
equal in number on the two surfaces, both seem to transpire 
alike ; and when neither possess stomata capable of action, the 
transpiration is scarcely to be observed. Again, if a plant actively 
transpiring under the influence of sun-light, be carried into a dark 
room, its transpiration is immediately and almost entirely checked; 
and if its stomata be then examined they will be found to have 
closed. Thus it appears almost unquestionable that the rapid 
loss of fluid from the whole vegetable surface, but especially from 
the leaves, which constitutes a most important part of the economy 
of the living plant, is regulated by the number of stomata which 
each part contains, and by the degree in which light acts upon 
them (§ 94.) 

258. Still, this kind of transpiration (which, to distinguish it, 
may be termed exhalation) is not altogether different in its cha- 
racter from the common evaporation first described. It will be 
recollected that the stomata open into large passages channelled 
out, as it were, in the fleshy substance of the leaf; and that the 
walls of these are every where composed of a very soft tissue, 
which is constantly kept moist by the crude sap conveyed so 
plentifully into the leaves. If, therefore, the atmosphere be ad- 
mitted into these passages, a very large amount of evaporation 
must take place from their sides, which resemble, in the want of 
any protection, the substance of plants habitually living under 
water; and this evaporation will be the more considerable, as the 



168 INFLUENCE OF LIGHT AND HEAT ON EXHALATION* 

surface exposed in these passages is much greater than that of 
the leaf itself. The exhalation of fluid from the living plant, 
then, may be regarded as a kind of evaporation from its interior, 
and will be promoted by the warmth and dryness of the air 
around ; but it is entirely controlled by the stomata, which, by 
admitting or excluding the air, permit or check it, in accordance 
with the influence of light upon them. 

259. Thus, then, we see one important mode in which light 
influences the growing plant. No amount of heat can supply a 
deficiency of this agent ; for, if it be excluded, exhalation is en- 
tirely prevented, and all the fluid that is transpired has to pass off 
by the slow process of evaporation only, which is not nearly 
sufficient for the required concentration of the sap. Moreover, 
when the exhalation is checked, absorption soon ceases ; for the 
tissues become gorged with fluid, and are capable of containing 
no more. If a plant, accustomed to grow in open day, be kept 
for some time in the dark, it becomes unhealthy, and, as it were, 
dropsical; and will generally die if not restored to its usual con- 
dition. This is not, however, the only process performed by the 
leaves, which is checked by the want of light; and therefore the 
unhealthiness which results cannot be imputed to it alone. 

260. We are now prepared to understand, then, the share 
which the leaves have in promoting and maintaining the absorp- 
tion of fluid by the roots. The exhalation which takes place in 
the leaves has a corresponding effect with the combustion of oil 
at the top of the wick of a lamp, — occasioning a continual de- 
mand for fluid from below. If the flame be extinguished, the oil 
does not flow over the top of the wick, because the absorption of 
it ceases also ; and so the action of the roots is governed by that 
of the leaves, the former organs ceasing to absorb the fluid, when 
it is not drawn off by the latter. This connexion is not only 
shown by the experiment just mentioned, but by a still more re- 
markable one, which explains in great degree the cause of the 
ascent of the sap in the spring, after it has been nearly stationary 
during the winter. If a vine be growing on the outside of a hot- 
house, and a single shoot be trained within, in the midst of 



ASCENT OF SAP. AMOUNT OF FLUID EXHALED. 109 

winter, the warmth to which the latter is exposed will cause its 
buds to swell and unfold themselves ; whilst those on the outside 
are quite inactive. A demand for fluid will thus be occasioned 
along this particular branch ; and this will be supplied by that 
existing in the vessels below. When these are emptied, they will 
be again supplied from the parts below them; and thus the 
motion will be propagated to that division of the roots whose 
fibres are connected with those of the vegetating branch, which 
will absorb fluid for its support, whilst all the rest are completely 
at rest. In the spring of the year, when the cheerful rays of the 
sun call the whole of the buds into activity, the whole of the 
roots are similarly affected ; and that the sap begins to move in 
the upper branches, before it commences ascending in the trunk, 
has been shown by experiment, — notches having been cut at in- 
tervals, by which the period of its flow could be ascertained in 
each part. 

261. Various experiments have been made at different times 
to ascertain the quantity of fluid thus exhaled by plants ; and 
the results of many of them are very interesting. Those made by 
Dr. Woodward 150 years ago have been already noticed (§ 100,) 
as indicating the large quantity of water absorbed. There is no 
great difficulty in ascertaining the amount upon a small scale ; 
for if a plant be supplied with a known weight of water, and the 
weight it has gained during a certain time be deducted from this, — 
allowance being also made for the evaporation from the surface 
of the water in which its roots are immersed, the quantity of 
which may be easily estimated, — the difference must be the pro- 
portion exhaled. This differs much in different plants according 
to the rapidity of their growth. 

262. It has been ascertained that the young loaves and shoots 
of the Wild Cornel exhale twice their own weight of water daily. 
A common-sized Cabbage in the twelve hours of daylight, was 
ascertained by Hales, (one oi the best experimenters upon this 
interesting subject) to exhale from 1.') to 25 ounces daily, accord- 
ing to the light and warmth to which it was exposed. This 
quantity, in proportion to the amount o[' surface exposed by the 

15* 



170 AMOUNT OF FLUID EXHALED. 

leaves, is probably as great as most plants furnish ; and is more 
than is given off from the skin of man in the same time. This 
has been reckoned to amount in twenty-four hours to about 25 
ounces ; which, as there is no great difference between day and 
night, would make 12s ounces in twelve hours ; admitting, there- 
fore, that the surface of his body is about one-fourth less than 
that of the leaves of the Cabbage, and reckoning the perspiration 
of the latter at the mean between the greatest and the least, it is 
still much greater than that of man. 

263. The transpiration of a Sun-flower in full growth, during 
fifteen days and nights, was carefully observed by Hales. This 
plant was 3§ feet high, weight 3 pounds, and the surface of its 
leaves was estimated at 5616 square inches, — or about 2| times 
that of the human body. The average transpiration during the 
whole period was found to be 20 ounces per day ; but in one 
warm dry day it was as much as 30 ounces. During a dry warm 
night, it lost 3 ounces — probably by simple evaporation ; when the 
dew was sensible though small, it neither lost nor gained ; and 
by heavy rain or dew, it gained 2 or 3 ounces. When this amount 
is compared with that perspired by man, it may be shown that, 
if their surfaces were equal, the man would perspire 50, and the 
plant 15 ; but that, for equal weights, the plant exhales 17, while 
the man perspires 1. Experiments upon single leaves, when not 
too long separated from the plant so as to lose their vitality ; yield 
fully as striking results. Thus a leaf of the Sun-flower weighing 
3 lil grains absorbed in four hours, by its petiole immersed in 
water, 25 grains of that fluid ; the leaf had increased in weight 
only 4s grains ; so that 20| grains had disappeared by exhala- 
tion. — Thus a quantity equal in weight to the leaf itself would 
have been exhaled in about six hours. 

264. Experiments of this last kind may be very easily per- 
formed by any one who has command of a pair of scales adapted 
to weigh small substances ; and it is well that the student should 
avail himself of such opportunities of learning how to " put Nature 
to the question " in matters of this simple character, in order to 
cultivate habits of accuracy and caution, which are useful in every 



EXPERIMENTS ON AMOUNT OF EXHALATION. 171 

condition of life. Let him take several leaves of different plants — 
such, for example, as the Vine, Oak, Elm, Beech, Lime, Apple, 
Pear, — weigh them separately, and estimate as nearly as he can 
the comparative surface presented by each. He would then place 
their footstalks in glasses or bottles of equal size, into which has 
been poured a certain weight of water, carefully ascertained to be 
the same in each ; and he would place all these in similar circum- 
stances for a certain time ; having also a corresponding glass with- 
out a leaf, in order to estimate the amount of fluid lost from the 
surface of the water by evaporation. By ascertaining how much 
had been absorbed by each leaf, and the weight each had gained, 
he would thus be easily enabled to, calculate the quantity it must 
have exhaled ; and then, by comparing this with the extent of the 
surfaces of the different leaves, he would estimate the proportional 
rapidity of the process in the various species he had chosen, — care 
having been taken to select, in the first instance, trees in equal 
stages of growth, and leaves of a similar degree of freshness and 
development. 

265. The watery vapour which is constantly though insensibly 
given off from the skin of animals, is liable to accumulate in 
drops, and to form sensible perspiration,, when from any cause it 
exceeds in quantity that which the air can carry away ; either in 
consequence of an increased secretion or separation of it from the 
blood (as when a person exerts himself in warm weather,) or from 
the atmosphere being already so loaded with dampness that it 
cannot contain any additional moisture. In the same manner, 
some plants exhale so rapidly at sunrise, when the heat of the air 
is not sufficient to enable it to carry off the disengaged moisture, 
that the fluid accumulates in drops at the points of the leaves, 
and has been mistaken for dew. This, however, is not the case; 
since it has been observed on plants under shelter, as well as on 
those that are exposed ; and it has been noticed also at other parts 
of the day. A similar accumulation of water in drops has boon ob- 
served when plants have been electrified ; by which process the 
amount of Exhalation appears, for a time at least, to be conside- 
rably increased. It is perhaps in this manner that an electric state 
of the atmosphere hastens the growth of sonic kinds of plants. 



172 VARIETIES IN DEGREE OF EXHALATION. 

266. If plants are exposed to a light of too great intensity, 
especially if they possess many stomata, and are not well supplied 
with water, their tissue becomes dried up by the increased exha- 
lation which then takes place, and which is not sufficiently coun- 
terbalanced by absorption, so that their vegetation is materially 
checked, — a fact of which we see abundant examples in dry 
sandy soils, and exposed situations. If, on the other hand, the 
leaves are shaded, and the roots freely supplied with moisture, 
the growth of the plant is active and luxuriant, but its tissue is 
soft and altogether destitute of firmness. This, however, is partly 
due to the imperfect performance of another process shortly to be 
described as that of digestion. Plants of a very fleshy juicy cha- 
racter, termed succulent, in which there is usually a great defi- 
ciency, or even entire absence, of stomata, require a considerable 
amount of light to secure for them that regular discharge of mois- 
ture which they require ; hence when Melons are grown in a 
frame, as many leaves as possible should be exposed to the influ- 
ence of the sun's rays, and the accumulation of moisture within 
should be provided against. There are certain succulent plants 
which, owing to their deficiency of stomata may be preserved 
without moisture for many days or even weeks ; and as their cu- 
ticle is so thick as to resist evaporation, it is often very difficult 
to kill and dry them for the purpose of placing them in collections. 
Of this kind are the Sedums, or Stone-crops, of Britain, which have 
been known to push considerable shoots when placed under pres- 
sure ; and many plants of tropical climates. 

267. Besides these applications of theory to practice, there 
are many others which will readily arise from the knowledge of 
the character of this function, and of the causes on which it is 
dependent. Thus we learn from it that the operation of trans- 
planting should not be performed in the summer, when the exha- 
lation is most active ; since, the roots being always injured in 
greater or less degree, the process of absorption is imperfectly 
performed, and cannot supply the loss by exhalation ; so that the 
plant is dried up. This evil may in some degree be guarded against 
by watering the plant copiously ; but it is much better to make the 



APPLICATIONS OF THEORY TO PRACTICE. 173 

change either in spring or autumn, when most plants are destitute 
of leaves, and therefore scarcely exhale at all, and when the func- 
tion is performed with much inferior energy in those which pos- 
sess them. 

268. Again, we see the reasonableness of the practice, which 
has been long known as a useful one, of keeping a nosegay, the 
freshness of which it is desired to preserve, in a dark room; since 
the check thus put to the exhalation which takes place, not only 
from the leaves, but also from the leafy surfaces of flowers, pre- 
vents the rapid withering which will otherwise occur. Even the 
light of lamps and candles is to a certain extent effectual in main- 
taining this function ; so that this is to be avoided where, for any 
particular occasion, flowers which have been picked are to be 
preserved as nearly as possible in their previous blooming state. 
Such a plan, however, would prevent the expansion of any buds 
which the nosegay might include ; since this (both in leaf-buds 
and flower-buds) depends upon the vigour with which the pro- 
cess is performed, and is hastened by light. By artificial light, 
therefore, it may be possible to cause flowers to bloom earlier than 
by any other means ; for it has been observed that the portions 
of a row of trees on which a strong light is cast from gas lamps, 
come into leaf sooner than the rest. 

269. The water exhaled by plants is very nearly pure ; so 
that what is furnished by different species varies extremely little 
either in taste or odour. It has been remarked, however, that 
fluid thus obtained becomes foul sooner than ordinary water; and 
this is the case wherever organic matter, even in extremely minute 
proportion, is diffused through the fluid. The quantity of solid 
matter contained in 40 ounces of the liquid, exhaled from a vino 
at the commencement of the summer, has been found to be only 
two grains; and no more than this was contained in 105 ounces 
from the same vine at the conclusion of the summer. Even these 
minute quantities are sufficient to communicate a perceptible 
taint, when separated by their decay into the gases o( which they 
are composed, and of which the bulk is very much greater. 



174 



ABSORPTION OF FLUID EY THE LEAVES. 

270. Although the leaves of the higher plants are, without 
doubt, the special organs of exhalation, they also have the power, 
like the leafy surfaces of the lower tribes of plants, of supplying 
fluid nourishment, under peculiar circumstances, when the sys- 
tem requires it. It has been already shown that it is only as we 
ascend the scale, that we begin to meet with distinct roots for the 
purpose of absorption ; — the same general surface answering both 
purposes in the simplest tribes. And even where distinct root- 
fibres are developed, they are often closely connected with the 
leafy surface ; these fibres being sent off from the under side of 
the leafy expansion of the Marchantia, and often from a similar 
part in the separate and delicate leaflets of Mosses. 

271. It is not to be wondered at, then, that the leaves of higher 
plants should be capable of supplying in some degree the func- 
tions of the roots, when these are absent or imperfect, or unable, 
from the nature of the soil which surrounds them, to obtain a 
sufficient supply of fluid nourishment. Xot unfrequently the 
roots, where they exist, serve merely to fix the plant, finding their 
way into the crevices of the hard dry soil or of the barren rock 
on which it grows ; and then it must be altogether dependent 
upon the moisture it imbibes through the general surface. In 
such cases the function of exhalation is but feebly performed ; 
and all the processes of growth are proportionably slow. Plants 
of this description flourish best near the sea-shore, where there is 
always a certain amount of moisture in the atmosphere ; and 
especially, too, in tropical regions, where the constant high tem- 
perature increases the evaporation from the surface of the water. 
and at the same time enables the atmosphere to dissolve a greater 
quantity of watery vapour. It is wonderful to see the precipitous 
faces of rocks apparently the most barren, studded here and there 
with plants of the Cactus and Orchis tribes ; the former exciting 
attention by the brilliancy of their blossoms, and the latter by the 
strange shapes they often present. In many of the Cacti growing 
in such situations, the stems contain a considerable amount of 



ABSORPTION OF FLUID BY THE LEAVES. 175 

fluid, which, though insipid, is wholesome, and is freely used as a 
cooling drink in fevers, by the natives of the countries in which 
these plants abound. 

272. Many of the Orchis tribe (as well as other plants) grow 
entirely in the air ; spreading themselves over the surface of trees, 
from which their roots hang freely down like fringes. Such attain 
their greatest luxuriance in the forests of the tropical parts of 
South America ; and here a constantly moist state of the atmo- 
sphere is maintained by the exhalation of the trees upon which 
they cluster. When grown in hot-houses in this country, they 
require that the atmosphere should be rendered artificially moist 
as well as warm ; and that their roots should be enabled to spread 
themselves freely through the air, and should not be confined 
within pots. The luxuriance which such plants often exhibit, 
sufficiently proves that the atmosphere contains all the materials, 
which are necessary for the growth even of the highest plants ; 
and that, if the structure is adapted to imbibe them from it, no 
other kind of supply is necessary. 

273. A fact similar to every one who has bestowed common 
notice on the processes of vegetation, equally proves that the 
leaves as well as the roots are capable of absorption. When 
plants are faded by the intense action of light and heat, and have 
suffered from deficiency of water, they are observed to revive 
rapidly when their surfaces are moistened, even if no fluid have 
been supplied to the roots. More precise experiments lead to the 
same result. It has been found that leaves placed with one of 
their surfaces upon water would remain fresh for several months ; 
the absorption through it counterbalancing the transpiration 
through the other. From a considerable number of experiments 
on different kinds (though this, again, is a subject which any one 
may investigate with great ease, and with the certainty of arriving 
at new and interesting results with a very little trouble,) it seems 
to be ascertained, that the leaves of trees and shrubs retain their 
verdure longest, when the lower side is placed in contact with 
water ; whilst the leaves of herbaceous plants absorb most readily 
by their upper surface, or in an equal degree by both. Thus. 



176 



DIFFERENT POWERS OF THE TWO SURFACES. 



leaves of the White Mulberry, placed with their upper side in 
water faded on the fifth day ; whilst those which absorbed by the 
lower surface remained fresh nearly six months. This effect, 
however, was no doubt due in part to the greater degree of ob- 
struction to the loss of fluid by transpiration in the second case 
than in the first ; the stomata being principally situated on the 
lower surface. But in experiments on other plants in which they 
are similarly disposed, the contrary result has been observed. 
Thus, leaves of the Nettle, whose inferior surface only was kept 
moist, faded at the end of three weeks ; whilst others whose upper 
surface was in contact with water, lived for two months. Lastly, 
the leaves of the Sun-flower, Kidney-bean, Cabbage, and many 
other plants were observed to remain fresh for the same length 
of time, by which ever surface they received their supply of fluid. 

274. This arrangement is admirably adapted for obtaining the 
greatest supply of the moisture contained in the lowest layer of 
the atmosphere ; for when, by the cooling of the earth's surface 
which takes place on a clear night, this moisture falls in the form 
of dew, it will manifestly be received on the upper sides of the 
leaves of plants which are but little raised above the ground ; 
whilst, on the other hand, the leaves of more elevated trees would 
not benefit by this deposition, being situated above its influence ; 
and these would receive and imbibe the vapour as it afterwards 
rises from the surface of the moistened earth. We thus learn 
that, if it be desired to revive drooping or sickly plants or trees 
by the application of moisture, the mode in which it should be 
distributed over them will depend upon their size ; if they be her- 
baceous plants, they should be watered from above ; and if they 
be tall shrubs or trees, the water should be thrown up by a syringe 
from below. 

275. The absorbing power of leaves has been shown by other 
satisfactory experiments . Some plants of Mercurialis (Mercury) 
were placed in water, some of them being immersed by their 
roots, and the others touching it by a part of their leaves alone, 
a small shoot of each being kept out, for the purpose of com- 
parison with the rest. After five or six weeks, the shoots of the 



NOURISHMENT OF PLANTS THROUGH LEAVES. 177 

plants which were nourished by the leaves differed little in vigour 
from those which had been supplied by the roots. Experiments 
upon single leaves, which have already partially faded, are still 
more striking. Some leaves of Potamogeton natans, (Pond-weed,) 
after being wiped dry, were weighed ; and after remaining out of 
the water for two hours, they were found to have lost from 3£ to 
51 grains each. They were then put in water, and after the lapse 
of two hours more, were again wiped dry and weighed. It was 
then found that they had severally gained from 3 to 5 grains 
each ; and this increase (which was also evident from the restora- 
tion of their natural freshness and plumpness) could only have 
taken place by absorption through the cuticle, as the cut ends of 
their footstalks were defended by soft cement. 

276. Other experiments show the remarkable influence of dew 
in supplying nourishment to plants. Two similar leaves of the 
Plant ago lanceolala (Ribwort Plantain,) equally faded, and each 
weighing 8 grains, were compared ; one having its footstalk im- 
mersed in water, and the other being exposed to dew. They both 
weighed 8 grains previously ; and on the following morning the 
first had gained but one grain, whilst the second (after the ad- 
herent moisture was wiped off) was found to have gained a grain 
and a-half. A similar experiment was then tried upon two leaves 
of Verbascum (Mullein,) each of which weighed 13 grains in the 
first instance. The one whose footstalk was immersed in water 
gained 2h grains ; whilst the other gained 4 grains. Many other 
experiments of a similar kind might be related ; but these are 
sufficient to show that leaves whose tissue has been deprived of 
fluid, have the power of replacing it by absorption from water 
placed in contact with them, or from a moist atmosphere. This 
power is probably exercised, however, in the majority of plants, 
only when their roots cannot from any cause obtain for them an 
adequate supply ; and at other times the leaves are organs of ex- 
halation only. 

277. The influence of dew and of a moist atmosphere in main- 
taining vegetation is often very remarkable in tropical islands. 
where no rain falls for months together, and where the soil is so 

16 



178 ABSORPTION OF MOISTURE FROM THE ATMOSPHERE. 

parched by the burning sun as scarcely to yield a particle of fluid 
to the plants growing upon it The proximity of the sea occa- 
sions the atmosphere of these islands to contain a large quantity 
of vapour, which, when the temperature of the soil falls at night, 
is deposited as an abundant dew ; and, in consequence, they ex- 
hibit a luxuriant vegetation under circumstances which would 
cause an inland country to appear completely parched. In the 
present year (1840) the preservation of the young corn during the 
hot and dry month of April, in many parts of England, was owing 
to the heavy dews. In consequence of the wetness of the pre- 
ceding autumn and winter, very little grain had been put into the 
ground before February ; and, as there was little rain from that 
time, the surface of the ground was not sufficiently moist to cause 
the rapid germination and growth of the young plants. Towards 
the middle of April the sun began to shine with great power ; and 
as no rain fell, it was much feared that the young plants, not having 
length enough of root to penetrate deep into the soil, would be 
starved for want of nourishment. This was supplied to them, 
however, in two ways ; — partly by the dews, which, in conse- 
quence of the clearness of the nights, were heavy ; and partly by 
the action of the powerful sun upon the deeper part of the soil, 
which had been drenched (as it were) with moisture by the rain 
of the preceding autumn, so that when it became heated, it sent 
up a large quantity of vapour, which was probably absorbed both 
by roots and leaves. 

278. This absorption of fluid by the leaves appears to take 
place chiefly through the membrane of the cuticle, but more par- 
ticularly by the downy hairs, which seem to act as so many root- 
fibres. They are chiefly developed, as already stated, upon plants 
which grow in situations in which they are much exposed to light, 
and to a dry atmosphere ; whilst the same species in damp shady 
situations will not form any. It has been noticed that they lift up 
their points and separate from one another at the approach of the 
evening dew, which collects in minute points around them ; and 
that they fall down again and form a layer of minute cavities on 
the cuticle, as soon as the heat of the sun begins to be perceived. 
On comparing the increase in weight when exposed to dew, in 



ABSORPTION OF DEW BY HAIRS. 179 

plants thickly furnished with hairs and possessing few or no sto- 
mata, with that manifested by plants having a smooth surface 
and many stomata, it is seen that the former is much the greatest ; 
and that it also surpasses in about the same proportion the weight 
gained by immersing the footstalk in water. Thus two heads of 
Marrubium vulgare (Common Horehound,) the original weight 
of which was 15 grains each, were placed, one with its stalk in 
the water, and the other in a place exposed to dew, for a night ; 
the first was found to have gained 2 grains, and the second 5 
grains. Both were exposed to dew during the next night ; and on 
the following morning they each weighed 23 grains, having both 
gained 8 grains, of which the first had acquired 6 in that night. 
A withered stem of Cerastium alpinum (Alpine Chickweed) 
weighing 5 grains, gained 6 grains by exposure to dew for two 
nights. 



ON RESPIRATION. 



279. The concentration of the crude sap by the loss of its su- 
perfluous fluid, and the occasional absorption of what may be 
necessary to supply the amount insufficiently afforded by the 
roots, are by no means the only functions of the leaves ; nor can 
they be regarded as the most important. These organs supply also 
the means of getting rid of a certain superfluous product, to retain 
which within the system (at least in the form in which it is set 
free) would be injurious and even destructive ; and they serve 
the equally important purpose of introducing, from the air, the 
element which chiefly gives firmness and solidity to the vegetable 
tissue. 

280. It is well known that, when an animal is confined in a 
limited quantity of air, it soon vitiates it, or renders it unwhole- 
some ; so that free ventilation, by which the foul air is replaced by 
that which is fresh, is one of the most important means of the pre- 
servation of health. Now this change in the air is effected by the 
removal of its oxygen, which is the element that chiefly supports the 
life of all beings; and by the substitution of carbonic acid gas set 



180 



RESPIRATION IN ANIMALS AND IN PLANTS. 



free from the lungs of the animal. Thus the blood is purified by 
the removal of a noxious ingredient, and rendered more capa- 
ble of maintaining the life of the system by receiving one of the 
opposite character ; and this change is manifested in its aspect as 
well as in its properties, — the dark purple blood of the veins being 
converted, by exposure to the action of the air in the lungs, into 
the bright scarlet fluid of the arteries. 

281. If the carbonic acid, which the blood takes up in its pas- 
sage through the vessels of the body, be not set free in this man- 
ner, in consequence of any obstruction to the admission of air into 
the lungs, or other similar cause, the animal dies. The throwing- 
off this superfluous ingredient is, indeed, one of the most constant 
of all the processes of the animal economy ; and there is good 
reason that it should be so, since it is set at liberty by the con- 
tinual decay to which all parts of the living body are more or less 
subject (the softer ones, however, much more rapidly than the 
hard ones,) and which is only prevented from becoming manifest, 
by the mode in which the decomposed particles are thus separated 
and carried out of the system, their places being supplied by 
others newly deposited from the nutritious fluid. 

282. Now this process of Respiration is as constant and uni- 
versal in the Vegetable Kingdom as it is in the Animal ; and it is 
only because a change apparently of a contrary nature sometimes 
obscures its effects, that it is not generally recognised. In fact, a 
healthy plant placed in a limited quantity of air, and exposed to 
the usual amount of daylight, will, (so long, at least, as it remains 
healthy,) add to its purity and will even restore the freshness of 
that which has been vitiated by an animal. But it is not the less 
true that there is a constant extrication of carbonic acid ; for this 
may be very easily proved to take place, even while carbon is 
being absorbed. If, for instance, a few small healthy plants be 
placed under a glass vessel from which all carbonic acid has been 
previously removed, and allowed to remain there even in sun-light 
for a few hours, they will be found to have set free a small portion 
of carbonic acid, which may be detected by shaking the air con- 
tained in the vessel with some lime-water, which it will render 



LIBERATION OF CARBONIC ACID IN GERMINATION. 181 

turbid. When the same experiment is tried in the shade, or by 
night, the quantity of carbonic acid found in the air is considera- 
bly greater. 

283. There are two periods during the life of a plant when this 
liberation of carbonic acid gas goes on with great energy. One 
is in the germination of the seed ; and here we can distinctly trace 
the object which is gained by the abstraction of the oxygen from 
the surrounding air, and by the conversion of it into this gas so 
opposite in its properties. In the seed when approaching maturity, 
a considerable quantity of starchy matter is laid up for the nourish- 
ment of the embryo; and this may remain unaltered for a long 
series of years, if the seed be not placed in those conditions which 
excite it to grow. But if it be exposed to warmth, moisture, and 
air (or any mixture of gases containing oxygen) it will sprout or 
germinate. In this process the starch, which (while it remains 
such) is unfit for the nourishment of the embryo that is being de- 
veloped, is converted into sugar, which forms an appropriate food 
for it. Had sugar been deposited in the first instance, it would 
have probably undergone fermentation, and would have thus lost 
its utility, before the time came for its office to be performed ; but 
the deposition of Starch, which can remain unchanged for almost 
any length of time, and which can at any time be converted into 
sugar, secures these objects in the most effectual manner. Starch 
differs but little from sugar in chemical composition, except in 
containing one additional proportion of carbon. When germina- 
tion commences, oxygen is absorbed by the seed, in the substance 
of which it combines with the carbon that is to be set free from 
it ; and a large quantity of carbonic acid is then given forth again 
to the air, whilst, in the same proportion, the starch is converted 
into sugar. 

284. It is in this manner that the nearly tasteless barley is 
changed into sweet malt ; and the change in the air around ex- 
hibits to us the function of respiration in its least complex form. 
Darkness favours it; since, as will presently be shown, a change 
of contrary character is favoured by light It is an Interesting 
fact that, after many trials, germination has been found to take 

16* 



182 RESPIRATION IN GERMINATION AND FLOWERING. 

place most readily in an atmosphere consisting of 1 part oxygen, 
and 3 parts nitrogen ; which is nearly the proportion of these in- 
gredients in the air we breathe. If the quantity of oxygen is 
much increased, the seed loses its carbon too rapidly, and the 
young plant is feeble ; and if the proportion is too small, carbon 
is not lost in sufficient amount, and the young plant is scarcely 
capable of being roused into life. 

285. The changes which take place in flowering are very 
similar to those occurring in germination. At the bottom of the 
flower is usually a fleshy expanded body, into which its different 
parts are inserted; this is called the disk or receptacle. Here, 
too, there seems to be a sort of reservoir of starchy matter for the 
nourishment of the newly-produced germs; which is converted, as 
in the other case, into sugar. A part of this is probably absorbed 
into the interior ; but the superfluous portion flows off in the form 
of honey. During the conversion of the starch into sugar, a large 
quantity of carbonic acid is substituted for the oxygen, which is 
absorbed ; and this appears to be principally effected by the in- 
terior organs of the flower. It has been found that an Arum- 
flower, whilst in bud, consumed in twenty-four hours 5 or 6 times 
its bulk of oxygen ; during the expansion of the flower, 30 times ; 
and during its withering, 5 times. When the outer leafy parts 
of the flower were removed, it was found that the oxygen con- 
sumed by the interior organs was much greater in proportion. In 
one instance, the stamens and pistil of an Arum consumed in 
twenty-four hours 132 times their bulk of oxygen. It has also 
been observed that double flowers, in which these internal organs 
are replaced by leafy parts, vitiate the air much less than the 
single flowers in which the former are perfect. 

286. The process of Respiration, then, in plants as in animals, 
appears essential to the life and health of the system ; and, though 
more energetic at some periods than at others, it is constantly 
performed. If a plant be surrounded by an atmosphere already 
vitiated, and be secluded from the influence of light, its respira- 
tion will be (like that of an animal in similar circumstances) so 
much impeded, that its speedy death will follow. But a very 



FIXATION OF CARBON FROM THE ATMOSPHERE. DIGESTION. 183 

different result occurs if it be exposed to strong or even moderate 
daylight. The carbonic acid of the air will be decomposed by 
the green parts of the surface of the plant, and the solid carbon 
will be fixed within its tissues, while the oxygen will be set free, 
so as to restore the purity of the air. It is in the performance of 
flhis function that the leaves, from the extent of the green surface 
they present, are peculiarly energetic ; for in that of respiration 
they only share with all the rest of the softer portions of the ex- 
terior ; and in fact the dark surfaces seem to have more to do in 
it than the light. 

287. Now this fixation of carbon, as it is termed, which 
antagonizes so remarkably the effects of the process of respiration 
may be regarded as in some degree analogous to the function of 
digestion in animals. In the solid food of all animals, whether 
it be of an animal or vegetable character, carbon is one of the 
principal ingredients ; and vegetables also require this, for the 
formation of their solid tissues. The proportion which they take 
in by the roots is but small ; and what they require in addition is 
obtained in this manner from the atmosphere, as they possess no 
stomachs by which it may be introduced in a solid form into 
the system. It is, then, chiefly of the water absorbed by the 
roots, and the carbon thus taken in by the leaves, that the ela- 
borated sap or nutritious juice of the plant consists ; and the con- 
stant liberation of carbonic acid from the general surface, in 
which the process of respiration consists, appears more necessary 
to preserve the healthfulness of the general system, by carrying-off 
what is in a state of commencing decay, than to change the 
character of this juice. 

288. The proportion of carbonic acidin which a healthy plant 
will thrive under the influence of strong sun-light, is not less 
than 7 or 8 per cent : but a much smaller proportion than this 
would soon be fatal to it if grown in the shade. It is to a pecu- 
liar compound formed in the cells of the given surfaces, of which 
the carbon introduced from the air is an essential ingredient, that 
the colour is due ; and as this fixation can only take place under 
the influence of sun-light, (artificial light, even the most powerful. 



184 EFFECTS OF DEFICIENCY OF LIGHT. 

having no influence on it,) plants which grow in dark situations 
are either in part or entirely destitute of colour. They are then 
said to be etiolated or blanched ; and the effect is purposely pro- 
duced in many instances. If the absorption of carbon from the 
atmosphere is checked, the fluids have a much more watery 
character, and do not contain their peculiar principles in nearly so 
much abundance. Hence many plants, which are rank to the 
taste and stringy in consistence, when growing in their natural 
conditions, may be rendered much more palatable by being 
blanched, — neither the peculiar secretions to which the rankness 
is due, nor the woody fibres which occasion its toughness, being 
then formed in the same degree. Thus it is that Celery, Sea-kale, 
and many other vegetables are blanched, — earth being heaped 
over their young shoots, so as to keep them from the light. As 
exhalation will also be checked by the same process, the 
tissue becomes distended with fluid, and acquires that succulence 
or juiciness which is so much valued in such vegetables. 

289. But though the bulk of plants which are undergoing 
this treatment, may considerably increase, yet the weight of their 
solid contents diminishes ; for during the whole period respiration 
is going on ; and, as there is thus a constant loss of carbon, whilst 
little or none is introduced, it follows that, if the tissues were 
dried by heat, they would shrink to less than their former amount. 
This is found to be the case ; and it is also true of a seed in the 
process of sprouting or germinating, which constantly diminishes 
in the weight of its solid contents, up to the time when some of 
the new-formed leafy surfaces become green, and begin to absorb 
carbon from the atmosphere. 

290. At this period of the growth of the young plant, it may 
be regarded as having a curious analogy with the tribe of Fungi. 
Both are supplied with nutriment already organized ; for whilst 
the one has it previously stored up by its parent, the other receives 
it from the decomposing matter upon which alone it can grow. 
Both are developed most rapidly and luxuriantly in the absence of 
light, if well supplied at the same time with warmth and moisture. 
And the Fungi, like the germinating seed, give out a large 



EXTRICATION OF CARBONIC ACID BY RESPIRATION. 185 

quantity of carbonic acid to the atmosphere, without receiving any 
carbon from it ; since the peculiar character of the aliment they 
imbibe by their roots renders any additional supply of this in- 
gredient unnecessary. In the Fungi, therefore, we have the 
process of respiration as distinct and easily understood as in 
animals, — both classes of beings subsisting upon food already 
organized, in which there is a large proportion of carbon ; and it 
is a little curious that the tissue of the Fungi should approach 
more nearly in chemical character to that of animal flesh than 
does any other vegetable substance ; so that, for those whose 
digestive powers are equal to them, Mushrooms constitute an 
extremely nutritious food. 

291. The process of digestion is confined, as before stated, to 
the leaves and those green surfaces of plants which correspond 
with them in function ; but that of respiration, although performed 
by the leaves more energetically than by any other part (at least 
during the ordinary processes of growth,) is not restricted to them, 
but is partially effected by the other surfaces, and even by the 
roots. The knowledge of this fact is important ; since, through 
ignorance of it, much valuable timber has been destroyed. Seve- 
ral years ago, during alterations in Hyde Park, a considerable 
depth of soil was added to a part of it in which grew some fine 
elm-trees ; the trunks of these were protected from pressure by 
circular walls built at a little distance from them ; nevertheless, 
the trees languished and died. Now the reason of this was sim- 
ply, that the roots, being covered with too great a depth of earth- 
could not exercise their usual function of respiration ; to perform 
which, they seem generally to direct their course as near the sur- 
face of the ground, as is consistent with the support they have to 
afford to the plant. 

292. Much discussion has taken place upon the question 
whether or not vegetation is upon the whole serviceable in purify- 
ing the air ; — that is, whether plants do altogether give out most 
carbonic acid or most oxygen to the atmosphere. By Priestley it 
was maintained that the latter was the only effect of vegetation . 
and that plants and animals are thus constantly effecting changes 



186 CONTRARY EFFECTS OF ANIMAL AND VEGETABLE LIFE. 

in the atmosphere which counterbalance one another. Subsequent 
experiments seemed to show, however, that the carbonic acid 
given out during the night might equal or even exceed in amount 
the oxygen given out by day ; but this was probably due to the 
employment of plants which had become unhealthy by being kept 
in a limited quantity of air, and which had not been exposed to a 
fair degree of light. For it has been recently shown by Dr. Dau- 
beny of Oxford, that in fine weather a plant consisting chiefly of 
leaves and stems, if confined in a capacious vessel, and duly sup- 
plied with carbonic acid during sunshine, as fast as it removes it, 
will go on adding to the proportion of oxygen present so long as 
it continues healthy ; — the slight diminution of oxygen and in- 
crease of carbonic acid, which take place during the night, bear- 
ing no considerable proportion to the degree in which the con- 
trary effect occurs by day. 

293. Thus we see that " the two great organized kingdoms of 
nature are made to co-operate in the execution of the same design ; 
each ministering to the other, and preserving that due balance in 
the constitution of the atmosphere, which adapts it to the welfare 
and activity of every order of beings, and which would soon be de- 
stroyed, were the operations of any one of them to be suspended. 
It is impossible to contemplate so special an adjustment of oppo- 
site effects, without admiring this beautiful dispensation of Provi- 
dence : extending over so vast a scale of being, and demonstrating 
the unity of plan upon which the whole system of organized 
creation has been devised." 

294. And yet man, in his ignorance, and his thirst for worldly 
gain, has done his utmost to destroy this beautiful and harmonious 
plan. It was evidently the intention of the Creator that Animal 
and Vegetable life should every where exist together ; so that the 
baneful influence which the former is constantly exercising upon 
the air whose purity is so essential to its maintenance, should be 
counteracted by the latter. Nothing is more prejudicial, therefore, 
to the health of a large population, than the close packing which 
too many of our cities exhibit ; hundreds of thousands of men, 
with manufactories of all kinds, the smoke and vapours of which 



PURIFICATION OF THE ATMOSPHERE BY VEGETATION. 187 

are still more injurious than the foul air produced by his own 
respiration, being crowded together in the smallest possible com- 
pass, with scarcely the intervention of an open space on which 
the light and air of heaven may freely play, and without any op- 
portunity for the growth of any kind of vegetation sufficiently 
luxuriant to give pleasure to the eye, or sufficiently energetic to 
answer its natural purpose. For the close confined air of towns 
is almost as injurious to plants as to animals ; the smoke which is 
constantly hovering above them, prevents their enjoyment of the 
clear bright sunshine which they require for their health ; and the 
fine dust, that is so constantly floating in the atmosphere, covers 
over their surfaces, and clogs up their pores. 

295. Hence the low stunted dingy vegetation, which the 
squares and open spaces of some of our large towns exhibit, is of 
little service; but extensive areas fit for the growth of lofty trees, 
are so beneficial in such situations, that they have been called 
the lungs of large cities, — so important is the purification of the 
air which they effect. It is true that they may occasion some 
degree of dampness in the immediate neighbourhood ; but this 
evil is much more than counterbalanced by the good they effect ; 
so that the cutting down of a tree in the midst of a large town, 
without some very strong reason, should be regarded as an of- 
fence not easily to be atoned for. It is much to be wished that 
the law of the land required such an open space to be set apart, 
whenever the population of an extending town or district increases 
beyond a certain amount. 

296. There is good reason to believe that Confervae and other 
aquatic plants exercise a similarly important influence, in keeping 
the water they inhabit in a state fit for the support of animal life ; 
since it appears probable that they absorb the products of the de- 
composition of that foul matter, by which all ponds and streams 
are constantly being polluted, and at the same time yield a supply 
of oxygen to the water. It is a fact well known that fishes are 
never so healthy in reservoirs destitute of aquatic plants, as in 
those in which they abound. The lower Cryptogamia appear to 
flourish better than higher plants would do, when supplied with a 



188 INFLUENCE OF INCREASED PROPORTION OF CARBONIC ACID. 

large quantity of carbonic acid, whilst the amount of light they 
receive is but moderate. In the lake Solfatara in Italy are seve- 
ral floating islands, consisting chiefly of Confervae and other cel- 
lular plants ; which are copiously supplied with nutriment by the 
carbonic acid, that is constantly escaping from the bottom of the 
lake with a violence which makes the water appear as if boiling. 

297. Under favourable circumstances, too, the highest plants 
are able to continue appropriating a larger proportion of carbon 
than that commonly existing in the air. The vegetation around 
the springs in the valley of Cottingen, which abound in carbonic 
acid, is very rich and luxuriant ; appearing several weeks earlier 
in spring, and continuing much later in autumn, than at other 
spots of the same district. But it is probable that, taking the 
average of the whole globe and of all seasons, the quantity of 
carbonic acid commonly existing in the air, is that which is most 
adapted to maintain the life of the race of plants at present inha- 
biting its surface, as well as to interfere as little as possible with 
the well-being of the animal creation. 

298. It is not improbable, however, that, in former epochs of 
the earth's history, a much larger amount of carbonic acid ex- 
isted in the atmosphere. At the time of the existence of those 
vast pine-forests, which, in their decomposed state, supply us with 
such an enormous amount of that most valuable article of Eng- 
land's wealth — coal — an article more really valuable to her than 
the mines of Peru,— scarcely any land animals seem to have ex- 
isted ; and these were of kinds which are now found to be ca- 
pable of breathing a comparatively impure air. The great lux- 
uriance of these forests, as indicated by the vast amount of their 
remains, and by more perfect specimens which have been pre- 
served to us, has led to the opinion that, at the period of their 
growth, more carbonic acid existed in the atmosphere than at 
present ; and that, in fact, it has from that time been gradually 
undergoing purification by the processes of vegetable growth ; 
and has at last become fit for the residence of the higher ani- 
mals. 



189 



RETURN OF THE ELABORATED SAP. 



299. The crude sap is brought into the leaves by the vessels 
which are connected with the woody portion of the stem ; and 
these branch out and subdivide in the veins, so as speedily to dis- 
tribute the fluid over the whole surface. After undergoing the 
various changes now described, it is collected again by a system 
of vessels which lie nearer the lower surface, and which communi- 
cate with the bark. These are principally of the kind formerly 
described as vessels of the latex; and through these the descend- 
ing sap, now completely changed in its properties is returned to 
the stem. This fluid contains the materials of all the products of 
the vegetable system ; — the elements of the organized tissues, the 
secretions which give solidity and toughness to the wood, those 
which occasion the delicious odours that so abound among plants, 
and those which supply so many useful and important products 
with which the comfort and luxury of man are largely connected. 
All these are entirely dependent on the action of the leaves; and 
the action of the leaves is dependent upon the supply of that 
amount of light and heat, but especially the former, which each 
species of plant requires. 

300. It not unfrequently happens that a plant will grow under 
a considerable change of circumstances ; but will not form its pe- 
culiar products in any thing like the same perfection as in its na- 
tural condition. Thus, Tobacco may be raised in this country, 
but it is far inferior to that of warmer and more sunny climes ; as 
is also the scent of the Rose, which does not here furnish enough 
of the very fragrant oil termed Otto of Roses, to make it worth 
while to cultivate the plant for this purpose, although the oil is 
imported from the East at an enormous price. On the other hand , 
the common Lavender is more fragrant in this country than in 
the South of Europe. In general, plants grown in wanner cli- 
mates, where the sky is less clouded, form secretions more active 
in every respect than those of temperate regions; — substances, 
for example, which are more powerful as medicines, or which have 
stronger and brighter colours, such as make them useful as dyes, 

17 



190 



DEVELOPMENT AND DEATH OF LEAVES. 




Fig. 44. Leaf-buds 
about to unfold; a, a, 



301. When leaves are first produced, they 
are small, very delicate in texture, pale in co- 
lour, and packed very closely together, form- 
ing what is called a leaf-bud. Some of the 
outer ones are of firmer texture and darker 
colour, and fold over each other like the tiles 
on the roof of a house, so as to protect the 
soft and delicate organs within. These are 
commonly termed scales, being often very 
different from the true leaves in aspect ; but 
there is no real distinction between them ; for, 
on opening the bud, it is easily seen that their 
inner layers gradually approach the true 
leaves, in appearance as well as in structure, 
and at last pass almost imperceptibly into 
them. The young leaves are most beautifully 
marks of the attach- folded together, in such a manner as to occu- 
above which buds are py the least possible space; and the peculiar 
always developed. mode in which this is done varies in different 
tribes of plants. Any one may examine it with the certainty of 
finding what will greatly interest him, by cutting across the leaf- 
buds with a sharp knife, when they are swelling, but before they 
have begun to expand. The outer scales are sometimes covered 
with a thick down, which may serve as a protection to them 
against the cold ; and sometimes they are coated over with a 
gluey substance, as in the horse-chestnut, which seems a very 
efficient guard. 

302. The young leaves in most leaf-buds may be easily ob- 
served to be arranged around a common centre or axis. When 
the bud lengthens, the insertions of the leaves, which were at 
first close together, are separated by the lengthening of the branch 
which bears them; and they then generally assume something of 
a spiral or rather a corkscrew-like arrangement round it, which is 
often very apparent. In fact this may be regarded as the regular 



ARRANGEMENT OF LEAVES UPON THE AXIS. 191 

mode in which leaves are arranged upon any part of the stem or 
branches of a tree. Starting from any one leaf, we shall generally 
find the next leaf not exactly above or below that one, but a little 
to one side of the perpendicular ; the next a little to one side of 
that; — and so on, until we come directly over the one from which 
we set off. We shall have thus made a spiral round the stem ; 
and the number of leaves we meet with in its course varies in 
different species of plants. Sometimes it amounts to twenty or 
more. Sometimes we only find two ; in this case each leaf is 
nearly on the opposite side of the stem from the other, but higher 
up or lower down. Leaves are then said to be alternate. The 
point of the stem from which a leaf originates is called a node; 
and the space between two nodes is called an internode. 

303. Now although it may be considered as the regular kind 
of growth, for a branch to lengthen equally throughout, yet we 
not unfrequently meet with varieties in the arrangement of leaves, 
occasioned by the cessation of growth at particular points. Thus, 
if the internode between any two alternate leaves is not developed, 
the leaves will be opposite to each other. Again, where each 
spiral turn contains several leaves, if all the internodes between 
the highest and lowest be undeveloped, these leaves will arise 
from the same point of the branch, still growing, however, in 
their proper directions; so that a complete circle of leaves, re- 
sembling that of the leafy parts of a regular flower, will be pro- 
duced ; this is called a whorl or verticil. There are some plants 
which exhibit the true spiral arrangement, as their regular mode 
of growth ; others in which we constantly find the leaves oppo- 
site ; and in some they are always verticillate. 

304. But there are many species which present differences in 
the arrangement of the various parts in the same individual, 
according to the circumstances under which each part has been 
developed ; and it is by such examples that we are able to discern 
the connexion between the several modes of growth. Thus, in 
the Rhododendron, we find the leaves sometimes opposite, some- 
times alternate. In the Honeysuckle, the leaves are naturally 
verticillate; but the whorlsare broken up, and the leaves carried 



192 INDEPENDENT LIFE OP LEAF-BUDS. 

to a distance from one another, by any thing which causes an in- 
creased development of the stem, just as when any leaf-bud 
(which has the young leaves arranged in a series of whorls, one 
above or within another) is elongated into a branch. On the 
other hand, in the Strawberry, the leaves which are usually 
alternate, become opposite or whorled at intervals. It is to be 
remarked that, when leaves are opposite, the several pairs are not 
in a line with one another above and below ; but each is at right 
angles to the next ; so that, if the internode between two pairs 
were undeveloped, a whorl of four leaves would be produced. 
Again, when one whorl is developed near another, their leaves do 
not issue from corresponding points in the stem, but are arranged 
in such a manner that the leaves of one arise from what seem to 
be the intervals of those of the other, so that the whorls are 
alternate to each other. The knowledge of this fact will be seen 
to be important, when the structure of the flower is described; as 
it will then be shown that its several parts are arranged upon the 
same principle with leaves. 

305. It is by the development of leaf-buds into branches 
bearing leaves, and capable of producing flowers and fruit, that 
the tree or plant is increased in size. The leaf-bud has also the 
power of developing roots, if removed from the parent, and may 
thus form a completely independent structure. It is by separating 
the buds, and by placing these in circumstances favourable to 
their growth, that any particular variety of plant may be pro- 
pagated more certainly than by seeds. As every bud is thus 
capable of maintaining an independent existence, it may be re- 
garded as in some degree a distinct individual ; and thus a tree 
would not be one being, but a collection of many. This is in 
part true ; still it must be remembered that, while all remaining 
upon one stem, they are almost entirely dependent upon it for 
nourishment, and are all liable to be influenced in the same man- 
ner, by any circumstances which effect it. 

306. Still it is quite possible for some buds to live while others 
die. Thus if arsenic be introduced into any portion of the sap- 
wood, it will give such a poisonous character to the fluid, that all 



DEVELOPMENT OF LEAF-BUDS. THORNS. 193 

the buds and branches in the line above it will be killed, the 
others remaining unaffected. It has even occurred that a single 
bud at the summit of a stem has preserved its life, whilst the 
vitality of all the others, and of the stem has been in some man- 
ner destroyed ; and that from this bud have been sent down 
bundles of root-fibres, between the bark and wood of the dead 
stem, which, when they have reached the ground, afforded abun- 
dant supplies of nutriment to the expanding bud ; and this has sub- 
sequently grown into a perfect tree, enclosing the original dead 
stem within its trunk. The original root-fibres are, in such a case, 
surrounded in the ensuing year by another layer more resem- 
bling wood, and this in the next season by another ; so that this 
portion of the structure, like the aerial roots of the Pandanus, may 
be regarded in the light either of stem or roots. 

307. Leaf-buds are always formed from the cellular portion 
of the stem or branches, on which the function of extending the 
growth of the individual seems especially imposed. They may 
be distinctly traced, in young branches, to the pith ; and where 
this has dried up, they may be seen to arise from the medullary 
rays. Sometimes they are stunted in their growth, and instead of 
being developed into branches, they remain as thorns,- which are 
neither more nor less than short pointed branches, containing 
much dense woody structure (by which they are rendered ex- 
tremely strong,) and being destitute of leaves. Any one may 
satisfy himself of this, by looking at the common Black-thorn, in 
which many intermediate conditions may be seen. Now under 
cultivation these undeveloped buds may be caused to become 
fertile branches; and this is another of the modes in which, in 
Linnaeus's phrase, " wild fruits " may be " tamed." (§ 23(5.) There 
are no thorns stronger than those of the Acacia tribe, which arc 
sometimes 5 or G inches long, formed with great regularity, and 
strong in proportion. The plants which bear them are often en- 
couraged to grow in the East, for the purpose of Forming hedges 
which serve most effectually to keep out intruders, unless these 
are covered with some almost impenetrable envelope. 

308. The influence of light upon the green colour oi' the 

17* 



194 INFLUENCE OF LIGHT UPON LEAVES AND STEMS. 

leaves is remarkably shown, when the buds are unfolding. The 
stronger the sunshine, the sooner will they assume their charac- 
teristic hue ; and, on the other hand, in dark dull weather they 
will remain for days together almost of the same colour as before 
they expanded. The following is an example of this fact more 
remarkable than is ever seen in this country. " It frequently 
happens in America that clouds and rain obscure the atmosphere 
for several days together ; and that during this time the buds of 
entire forests expand themselves into leaves. These leaves as- 
sume a pallid hue until the sun appears ; when, within the short 
period of six hours of a clear sky and bright sunshine, their colour 
is changed to a beautiful green." One writer mentions a forest 
on which the sun had not shone for twenty days. " The leaves 
during this period were expanded to their full size, but were almost 
white. One forenoon, the sun began to shine in full brightness ; 
the colour of the forest absolutely changed so fast that we could 
perceive its progress. By the middle of the afternoon, the whole 
of this extensive forest, many miles in length, presented its usual 
summer dress." 

309. The influence of light is also shown in modifying the 
direction of the stem. Where a plant sends forth a single stem 
or shoot, it will always direct itself towards the light; and this is 
especially manifested where the light comes in only one direction, 
as when a Potatoe, which has begun to grow in a cellar, sends a 
shoot of several feet in length towards any aperture through 
which even a small quantity of light finds admission. The rea- 
son obviously is, that, in consequence of the loss of fluid from the 
tissue of the stem, on the side on which the light falls, it is con- 
tracted, whilst that of the other side remains turgid with fluid; the 
stem makes a bend, therefore, until its growing point becomes op- 
posite to the light, and then increases in that direction. Should 
any obstacle divert it, the same cause will bring it back. 

310. The direction of the branches of trees growing in open 
spaces is less influenced by light ; since the rays of the sun exert 
a much inferior power in this respect, when they are reflected 
from clouds and other objects, so as to form what is commonly 



INFLUENCE OF LIGHT UPON DIRECTION OF STEM. 195 

termed " diffused daylight." It is a curious fact, however, that 
there seems a tendency in almost every growing tree to send its 
principal trunk directly upwards. Experiments have been made 
for the purpose of changing this. For instance, the stem has 
been bent much out of the perpendicular by means of a rope tied 
round its summit, and has been kept so for a long period. The 
plant would then push up that one of the side-shoots which is 
most nearly in the line of the trunk ; and this, increasing in 
length and diameter in successive years, will gradually present 
more and more the appearance of a continuation of the lower 
part of the stem, whilst that which was bent down presents the 
aspect of a branch. 

311. The same means is adopted to repair a natural injury. 
Thus, the upper half of a large elm tree, which had a long straight 
trunk, the lower half being without branches, was broken off by a 
violent gust of wind. From the complete absence of leaves in the 
trunk that remained, it was not expected to survive ; but, being 
in full sap at the time, the abundant nourishment it contained oc- 
casioned the development of buds which were previously inactive, 
and a great number of small branches soon issued from the stump. 
Of these, the upper ones have grown most rapidly; and the two 
highest which were at first nearly horizontal, have gradually 
changed their direction, so as to follow the line of the upright 
stem ; and it now appears as if the trunk had originally divided, 
at that point, into two minor ones, — so completely has all appear- 
ance of the accident been lost. 

312. The leaves of all plants have a very limited term of ex- 
istence. In temperate climates most trees shed them during the 
autumn, and pass the winter in a state of complete inactivity. 
Before they fall off, the leaves usually change colour, — sometimes 
very decidedly, as does the Beech ; and it has been ascertained 
that at this period they absorb more oxygen, and give out more 
carbonic acid, which indicates their commencing decay. This 
absorption of oxygen has been shown by experiment to be the 
immediate cause of the change of colour ; since the green matter 
of leaves, when acted on by substances which readily yield oxygen. 



196 CHANGES OF LEAVES. 

is found to exhibit it. The separation of the leaf from the stem 
is probably due to several causes. During the latter part of the 
summer, some of the vessels become choked up with solid matter, 
and those which proceed from the interior of the trunk are over- 
strained by the addition to its diameter which has taken place, so 
that they are easily ruptured. The tissues of the leaf itself, too, are 
gradually dried up ; and the whole structure loses its vitality, and 
is cast off as a dead part is from the body of an animal. 

313. Trees and shrubs which are spoken of as evergreens, do 
not really retain their leaves for more than a year ; but they are 
not cast off until a new crop appears, and the exchange does not 
take place suddenly but gradually, so that the aspect of the tree 
never undergoes much alteration. In evergreens, the functions of 
the leaves are carried on, though with great languor, during win- 
ter ; but at other parts of the year they are less active than those 
of the species which lose their leaves in autumn. There are some 
trees of tropical climates which completely lose their leaves two or 
three times in every year, appearing as bare as in winter ; and 
these are speedily replaced by a new crop. It is probable (though 
it has not been certainly ascertained) that in such trees, a new 
woody layer would be formed by every crop of leaves. In very 
hot and dry summers of this country, trees have been completely 
stripped of their foliage early in July, and have had strength to 
put forth a new and apparently vigorous crop of leaves. Such an 
effort, however, appears very exhausting to the tree, which is 
seldom so vigorous the next year. 



314. The facts stated in this chapter respecting the influence 
of Vegetation on the surrounding air, are very interesting in con- 
nexion with a plan which has recently been practised with much 
advantage, of growing plants under glass cases very nearly closed. 
They are constructed of almost any form or size ; — the experiment 
will answer with a chamber of a foot in each direction, or in one 
as large as a common green-house. The plants to be reared are 
placed in a kind of trough, with a sufficient quantity of light earth 
and water ; and the glass cover is then fitted upon this in such a 



GROWTH OP PLANTS IN CLOSED GLASS CHAMBERS. 197 

manner as not to be quite air-tight, but to allow of extremely little 
communication between the interior and the surrounding air. The 
advantage of this system is, that the plants are kept in an atmo- 
sphere thoroughly saturated with moisture ; and that they can ob- 
tain the necessary supply of carbonic acid (which finds its way 
through the crevices beneath the glass cover) without being also 
exposed to the impurities which the atmosphere contains. In 
London and other large towns, the air is loaded with particles of 
soot and dust, which are so injurious to vegetation, that none but 
the most hardy plants will flourish there without some protection ; 
yet under this system, the most delicate and beautiful plants may 
be reared, provided they are sufficiently supplied with light and 
warmth. Again, many plants are killed by exposure to the sea- 
air during long voyages ; this being loaded with particles of salt. 
But under this protection, plants have been successfully trans- 
ported to England from the most distant quarters of the globe, 
which could only be previously introduced by seeds. By the 
adoption of this plan, many who take an interest in the cultivation 
of plants too delicate for our own climate, may indulge their taste 
at a small expense. A window should be selected with a southern 
aspect, or nearly so, and a second pair of sashes should be fixed 
at the distance of a foot or 1 8 inches within or outside the first, as 
may be most convenient, — the fitting of the whole being as tight 
as possible, consistently with the easy movement of the inner 
sashes. In the space at the bottom, a trough is placed contain- 
ing moistened earth ; and the plants grown in it should be so 
trained as to expose the surface of their leaves to the light as freely 
as possible. If a winter warmth is required, the window of a sit- 
ting room in which a fire is constantly maintained, should be 
selected ; and this will suffice to grow many plants which naturally 
inhabit the warmer regions of the globe. This method is particu- 
larly adapted for the growth of Ferns.* 

* The public arc indebted tor this important system to Mr. N, B, Ward, 
of London. The author has that gentleman's authority tor Baying, that 
the common notion that the external air should be completely excluded, is 
quite an erroneous one. 



CHAPTER IX. 

GENERAL REVIEW OF THE NUTRITIVE PROCESSES IN PLANTS. 

315. The functions of the several organs concerned in the 
Nutrition and Growth of the plant having now been separately 
described, it next becomes desirable to take a general review of 
the whole, and to trace the connexion between their respective 
actions, and to point out their bearing on the object of the whole 
of this beautifully-arranged system. When we look at a well- 
ordered household, we observe that the actions and duties of each 
member of it are planned and arranged by the heads of the family, 
so as to accord with their respective qualifications, and at the same 
time to conduce to the comfort and happiness of the whole ; and 
the more completely this is accomplished, the greater is the har- 
mony and regularity with which the labours of the whole are per- 
formed, — the less is the liability to interruption, arising either from 
the caprice or incapacity of any of the labourers. And in pro- 
portion to the completeness with which this end is gained, do we 
think highly of the wisdom of those who have studied and exe- 
cuted the means of attaining it. 

316. Now the economy* of the vegetable is precisely analo- 
gous to that of such a household. The whole structure is com- 
posed of a number of different organs or members, having different 
parts to perform in the general scheme ; and these parts or func- 
tions are so beautifully adjusted together, that, in every variety of 
circumstances in which the being is liable to be placed, they shall 

* This word literally signifies household-la to; and in this sense it is ap- 
plied in physiology, to designate the regular harmonious system un which 
the actions of living plants and animals are performed. 



CONVERSION OF THE FOOD OF PLANTS. 199 

still be executed in harmony, and with one common purpose. 
Thus we have seen that one organ pumps up the required water, 
another carries it, another uses it in cooking, another gets rid of 
the waste, another obtains the solid food, another carries the 
cooked provisions to all parts of the structure, another stores up 
the superfluity, another builds additions to the edifice, whilst ano- 
ther prepares to send out a colony, furnished with supplies or food, 
and with every thing requisite to begin life for themselves. Now 
we have considered the separate parts of the establishment — we 
have inspected the pump, the conduits, the kitchen, &c; the gene- 
ral economy of the whole remains to be reviewed. And if we 
have seen a Wise Design in those, our ideas will be still farther 
elevated by this mode of viewing the subject. 

317. We have seen that the fluid absorbed by plants consists 
of water, in which are contained carbonic acid and ammonia ; 
and in which are also dissolved the various mineral substances 
which each species requires for its healthy existence, but which 
contribute nothing to the formation of those peculiar organic sub- 
stances that compose the vegetable tissues. The conversion of 
these elements into the substances intended for the nourishment of 
the plant begins very low in the stem ; and the proportion of them 
increases as it ascends. The substances at first produced are gum 
and sugar, which are the simplest in their chemical nature of all 
organic compounds ; being made up of oxygen and hydrogen in 
the same proportions as water contains, with the addition of car- 
bon. Being nearer inorganic bodies in their composition, and in 
the tendency of the latter to the crystalline form, they are also 
nearer in properties ; for they may be preserved for years in a dry 
state, without any impairment of their characters, since their ten- 
dency to spontaneous decomposition is not greater than that of 
many mineral substances. 

318. The crude sap immediately that it has been absorbed, 
begins to mix with the matter which has been stored up from the 
previous year, in the tubes and vessels through which it rises; and 
in proportion as it ascends the stem, it dissolves more and more 
of this, so as gradually to present in taste, odour, &c, the peculiar 



200 CONVERSION OF THE ASCENDING SAP. 

characters of the plant itself. The object of this admixture has 
been already stated (§ 119,) and it also probably has another. It 
seems to be a general law of the organized world, that the raw ma- 
terial taken in as an aliment by any being, cannot be assimilated 
(that is, converted into the nutritive matter required to repair, re- 
new, or extend its tissues,) without being mixed with matter 
previously assimilated. Thus, in animals, the chyle which is 
taken in from the food in the intestines, and which contains the 
materials of blood, is not sent to nourish the system until it has 
been mixed with the blood previously circulating, and acted on 
by it. In plants this object is effected during the period of active 
vegetation, by the admixture of a portion of the descending sap 
with that which has been just absorbed ; but as all activity ceases 
during the winter, the same object is provided for in the autumn, 
by causing the essential parts of the sap to be deposited in the 
tissues, so as to be dissolved in the ensuing spring. 

319. Although the peculiar characters of the proper juices of 
the tree are thus communicated to the ascending sap, yet they are 
possessed by it in but a very slight degree ; and it is in conse- 
quence of this, that the ascending sap of all trees possesses very 
nearly the same properties; — as is shown by more than one cu- 
rious fact. There are some plants which have not the power of 
forming true roots for themselves, and which obtain their supply 
of sap from the stems of trees to which tbe)^ attach themselves. 
Such is the common Misseltoe. The seeds of this plant are de- 
posited by birds on the exterior of the stems and branches of 
trees; and the root-fibres which they put out, insinuate themselves 
through the crevices of the bark, and incorporate themselves with 
the wood. 

320. Now the Misseltoe imbibes the ascending sap from the 
wood of the tree or stock on which it grows ; and this it converts 
into a proper juice, adapted to nourish its own structure, by means 
of its own leaves. The ascending sap of most trees being nearly 
alike, the Misseltoe seems to grow with almost equal facility on a 
great variety. It is remarkable, however, that it is very rare on 
the oak ; and it is perhaps this circumstance, which caused the 



PARASITIC GROWTH OF THE MISSELTOE. 201 

plant when found in connexion with that tree, to be regarded by 
the ancient Druids in a religious light Perhaps it is the tannin 
produced by the oak, of which a small portion will be contained 
in the ascending sap, and which has been already spoken of as 
exerting a prejudicial influence on the vegetation of most other 
species (§ 209,) which is unfavourable to the growth also of the 
Misseltoe. 

321. Now it is a very curious fact, that the law of growth of 
these root-fibres is different from that which governs other roots. 
For whilst the latter grow downwards towards the centre of the 
earth, these grow towards the centre of the bough or stem into 
which they may be penetrating. This tendency was ascertained 
by the experiments of the French physiologist Dutrochet, who 
caused a seed of Misseltoe to germinate when hung by a thread 
near a large ball of metal; and he found that the radicle always 
directed itself towards the centre of this ball, near whatever part 
of the surface it might be placed. By this curious adaptation, the 
Misseltoe, which, from the want of power to form perfect roots, 
would otherwise be unable to exist, is endowed with a compen- 
sating power;— it being as much a part of its natural habits to 
grow upon the stem and branches of trees, as it is for other plants 
to send their roots down into the ground. 

322. The fibres of the Misseltoe seem to incorporate them- 
selves completely with those of the stock ; and so intimate is the 
connexion between them, that coloured fluids will pass from the 
stem into this natural graft, — for so it may be termed. It does 
not appear, however, that any communication exists between the 
parasite and the bark beneath it, which is always found to be in 
a dead state around its insertion. But if the part of the branch 
at Which it penetrates be divided with a saw, it will be seen that 
the two woods are so thoroughly united, that the line oi' separa- 
tion between them can scarcely be traced. That the Misseltoe is 
itself quite deficient in the power of absorbing fluid, has been 
clearly proved by experiment. If the stem of this plant be cut 
off and immersed in water, it will absorb little or none of the 
fluid; whilst, if a portion of the branch with which it is connected 

18 



202 CONVERSION OF ASCENDING SAP. 

be cut off and immersed in a similar manner, it will absorb nearly 
as much as if furnished with leaves of its own. 

323. A curious fact illustrative of the great difference in the 
characters of the ascending and descending sap, is that the former 
is nearly or quite harmless in those plants whose proper juices 
have the most virulent properties. Thus, the inhabitants of the 
Canary islands draw off the former, which serves as a refreshing 
drink, from the interior of the stem of the Euphorbia canariensis; 
a tree of which the descending sap is of a very acrid nature, re- 
sembling that of the common Spurge of this country, but much 
more powerful. 

324. The conversion of the water and carbonic acid absorbed 
by the roots into gum or sugar, involves the setting free a portion 
of the oxygen contained in those compounds ; for as water is com- 
posed of oxygen and hydrogen, and carbonic acid of oxygen and 
carbon, it is evident that, in the production of any substances con- 
taining no very large proportion of oxygen combined with carbon 
and hydrogen, there must be a superfluity of the first of these in- 
gredients. This oxygen is probably conveyed away by the spiral 
vessels which form the medullary sheath of Exogens, and which 
are diffused through the woody bundles of the stem of Endogens. 
These spiral vessels communicate with the leaves ; and through 
them, the oxygen is given off to the atmosphere. By dividing 
stems under the surface of mercury,* and collecting the minute 
bubbles of gas which arise from the cut ends of the spiral vessels, 
it has been shown that they contain a considerably greater pro- 
portion of oxygen than exists in the atmosphere. 

325. The processes by which the crude sap conveyed to the 
leaves is converted into the " proper juice" of the plant, have 
already been described in so much detail, that it is not necessary 
to do more than briefly recapitulate them here. By Exhalation 
and Evaporation a great quantity of superfluous water is got rid 

* This fluid is used instead of water, when it is desired to collect gases 
which might be absorbed by water. In the present case it is employed 
to prevent any carbonic acid that might exist in the vessels, from being 
undiscovered, through the absorption of it by the water. 



FORMATION OF PROPER JUICE. 203 

of; and the fluid is thus concentrated. By the absorption of car- 
bon under the influence of light, — the process to which the name 
of Digestion may be given, — a large quantity of solid matter is 
added to it ; and the materials are afforded for the increase of the 
woody structure, which requires this ingredient in a peculiar de- 
gree. And by the process of Respiration is removed the product 
of the slow decay of the whole structure, which would be highly 
injurious if retained within it. 

326. We must remember, however, that, in speaking of these 
changes, we only state the evident results of those more obscure 
ones which take place in the interior of the plant, and of which 
the nature is still unknown. In all plants, the functions of Exha- 
lation, Digestion, and Respiration, are performed almost in a 
similar manner ; and the materials upon which they operate are 
(as already explained) nearly of a similar character ; and yet the 
products are remarkably different. In nearly all of them, how- 
ever, the material of the tissues themselves is the same. Woody 
fibre, for example, is found by the chemist to be composed of the 
same elements, in the same proportions, from whatever tree it is 
taken, provided it be cleared from those substances which are 
deposited within it for the purpose of affording it additional 
strength. But when we look at the immense variety of products 
which the vegetable kingdom supplies, varying no less in proper- 
ties than in appearance, we are lost in wonder at the marvellous 
nature of those processes, in which a difference, undiscoverable 
by all our most refined means of research, shall be productive of 
such a number of widely-different results. And at the same time 
the reflecting mind cannot forget that these results are all of a 
kind most valuable to Man, furnishing him with the necessaries, 
the comforts, and the luxuries of life, — support in health, medicine 
in disease, and the materials of great part of his clothing, his 
books, and various articles which minister to his mental and 
moral improvement. 

327. These substances may be distinguished under two 
classes;— the nutrit ivc products, adapted to supply the materials 
of increase to the tissues themselves ; — and the special secret io)is. 



204 NUTRITIVE PRODUCTS OF PLANTS. GUM. 

which are for the most part contained or stored up in them. The 
former are common to all plants ; of the latter different kinds 
exist in the various tribes. 

328. Of the nutritive products, which are carried by the de- 
scending sap to all parts of the structure, (as are those of a similar 
nature contained in the blood of animals,) the principal is Gum. 
This is found in the bark and wood of all plants ; and is present 
in such abundance in several, which are called Gum-trees, as to 
flow in plenty from the bark when wounded, or when its surface 
cracks. Of these trees, most belong to the Acacia tribe ; and it 
is in warm climates only that the formation of this product is so 
abundant as to make the collection of it desirable. Various mo- 
difications of this principle exist in different vegetables ; but they 
may all be regarded as combinations of pure gum with other sub- 
stances. Gum Arabic is one of its simplest forms ; this is really 
brought from Arabia, where it is annually collected in the Acacia 
forests, at the end of November. A large quantity is imported 
into this country, on account of its extensive use in calico-print- 
ing and other arts. It is a highly nutritious substance to man 
and animals ; and it forms an important article of diet in Arabia 
and Senegal. Those who are engaged in collecting it live for a 
time almost entirely upon it ; and six ounces have proved suffi- 
cient to support an adult for 24 hours. It is on record that a 
caravan crossing the Desert, their provisions being exhausted, 
preserved themselves from famine by eating the Gum Arabic 
which formed part of the merchandize they were transporting. 
But no animals could continue long to subsist on this ingredient 
alone ; since it contains no nitrogen, which is still more essential 
to their support than to that of plants. 

329. Gum is almost the only organic substance that seems 
to be immediately applied to the nutrition of the plant, when 
absorbed from without, instead of being first decomposed into 
water and carbonic acid ; for a plant thrives well in a solution of 
it. This is evidently because it thus supplies an important ingre- 
dient in the ascending sap, in which it would otherwise have to 
be formed. (§ 317.) The gum contained in the elaborated sap 



NUTRITIVE PRODUCTS OF PLANTS. GUM. 205 

appears to have undergone some change, which renders it more 
prepared for being converted into an organized tissue. It is this, 
which, being poured out between the bark and the newest layer 
of wood, is the viscid substance termed cambium; in which the 
rudiments of the cellular tissue, that is to form part of the new 
layer of wood, after a time present themselves. Even if this cam- 
bium be drawn off from the stem, its particles show a tendency 
to arrange themselves in a form resembling that of cells and ves- 
sels ; though no perfect tissues are produced by this kind of co- 
agulation. The interior of young seeds is filled with a glutinous 
pulpy fluid of a similar description; and partitions gradually ap- 
pear in this, converting it into a mass of cellular tissue. 

330. If a wound be made in the bark, a similar glutinous exu- 
dation is thrown out from the cut edges; and by the conversion 
of this into solid tissue, the wound is gradually healed. If a com- 
plete ring be cut away from the bark, this exudation will be much 
the greatest on the upper side,— showing that it comes from the 
descending sap ; but it is not altogether confined to that edge since 
a portion of the descending current, having been carried by the 
medullary rays into the interior of the stem, is not checked by 
this interruption to its flow through the bark. Thus we perceive 
that, although there is not in Plants, as in Animals, a regular con- 
tinuous circulation of nutritious fluid, — that which has once passed 
through the system of the latter, being impelled again through its 
vessels, after having undergone the necessary purification, — Na- 
ture has provided for the reparation of their wounds in the most 
advantageous manner. 

331. From this form of Gum it would appear that the ma- 
terials of cellular tissue are produced ; but those of woody fibre 
are not the same in chemical constitution, containing a larger 
proportion of carbon. And thus we see why it should be pecu- 
liarly necessary for the production of woody fibre, that the leaves 
should be exposed to the full influence of light, by which alone 
the proper amount of carbon can be introduced into the system. 
As already stated, whilst cellular tissue increases in every direction, 
woody fibres seem to grow almost exclusively downwards. They 

18* 



206 FORMATION OF TISSUES FROM GUM. — SUGAR. 

may be traced gradually descending from the leaves, in which 
they always originate, just as the roots make their way through 
the earth. They pass down in the space between the bark and 
wood, at the time the cambium is there ; and this fluid probably 
contains the materials for both tissues. If an obstacle intervene, — 
as, for example, a branch passing off from the stem, — they do 
not stop in consequence of it, but separate to one side and the 
other, and re-unite below, just as a bundle of roots would have 
done. These fibres, being intermixed with the cellular tissue 
produced by the cambium, compose the new layers of wood and 
bark, of which a new one is formed every year ; and it is in this 
way that those additions are made to the quantity of solid matter 
contained in the stem, which the supply of descending sap is 
principally intended to furnish. 

332. The production of new buds is accomplished, as already 
stated, by the cellular tissue alone ; and as they are connected 
more or less closely with the medullary rays, it is easy to under- 
stand how they derive their nutriment from the descending 
current. Nothing but cellular tissue exists in them, until they 
have expanded themselves into true leaves, and then they form 
the materials of woody fibre for themselves. The same is the 
case with flower-buds, seeds, and other young parts. The sub- 
stance termed pectin, which constitutes the jelly of fruits, is very 
closely allied to gum, and may be converted into it. 

333. Although Gum seems to be the chief nutritious product 
of the assimilation, by the plant, of the substances which formed 
its aliment, it is not the only one. Sugar in many cases appears to 
have the same office, especially in young and rapidly-growing 
parts. Thus, the starch of seeds is converted into sugar in the first 
stage of their growth (§ 283.;) and the sugar is dissolved by the 
water around, and carried up the young stem to the leaves. The 
starch existing in the disk of flowers, again, is converted into sugar 
for the nourishment of the young seeds ; and it is the superfluous 
portion of this which flows off in the form of honey. There are 
particular plants which contain a very large proportion of sugar, 
just as we have noticed others which abound in gum. Such are 



NUTRITIVE PRODUCTS OF PLANTS. SUGAR. 207 

the Sugar-Cane, the Beet-root, and the Maple. The sweet juice 
which abounds in the Sugar-Cane is exhausted by flowering, and 
appears, therefore, destined for the development of the set of 
organs concerned in that process. The same is the case with the 
Beet-root, and also in the Maple; in the former, the sweet juice 
does not begin to accumulate in the roots, until the development 
of the growing parts has ceased for that year ; in the latter, the 
juice which was previously sweet ceases to be so whilst the tree 
is putting forth its buds, leaves, and blossoms ; in both these in- 
stances, the use of the sugar in the vegetable economy is clearly 
seen. 

334. Of the importance of Sugar as an article of commerce 
little need be said. The annual production in different parts of 
the world is estimated at not far from 20 million hundred-weights, 
or a million of tons ; and this is nearly all obtained from a single 
kind of plant, — the Sugar-Cane. The soft spongy tissue of this 
plant, previously to its maturity, contains a large quantity of a 
sweet juice, which is pressed out from the stems by passing them 
between rollers. This juice is boiled down into a thick sirup, 
which crystallizes and deposites the sugar it contains. This is what 
is commonly known as brown sugar ; and it has to undergo a 
subsequent process of refining, in order to convert it into white. 
In Canada and other parts of North America, a good sugar is 
produced from the Maple, by tapping the stem when the sap 
begins to arise in the spring ; the quantity of sugar obtained, by 
boiling the sap that flows from one tree during a period of six- 
weeks, is sometimes as much as 30 lbs. 

335. It is not unfrequently necessary that a store of nutritive 
matter, which may be required at some future time, should be pro- 
vided in the Vegetable system, in such a situation that it shall be 
out of the general current of the circulation, and at the same time 
easily brought into it. In animals, the/*// constitutes a store of 
this kind. The superfluous nutriment introduced into their system 
is converted into this substance ; which, besides other purposes 
that it serves, is ready for the support of the body, when from am 
cause there is a failure of the supply on which the animal usually 



208 STRUCTURE AND OFFICES OF STARCH. 

depends. In some animals, this production of fat takes place at 
regular periods ; thus Bears, which pass nearly the whole winter 
in sleep, and take little food during that season, become very 
plump in the autumn, and are observed to be very lean soon after 
they have emerged from their winter retreat. 

336. Now the Starch which is found so abundantly in many 
plants, and in some part of almost every one, serves the same 
purpose as fat. It is gum, slightly altered, and enclosed, as it 
were, in a series of minute bags, which fill the cells of cellular 
tissue and receive their form. Starch, when removed from the 
plant, exists in the form of minute granules ; each of which, when 
examined with the microscope, is found to consist of a series of 
layers of a half-fluid substance, the interior ones being nearly 
fluid like dissolved gum, and those on the outside being almost as 
firm as membrane. When put into cold water, they retain their 
structure, as the outside layer is not acted on by that fluid ; but 
when exposed to a heat of about 160°, this little sac bursts, and 
its contents are set free and dissolved in the water ; and this is 
why starch, once dissolved in hot water, can never be restored 
to its original form. 

337. Thus, then, we may consider starch as little else than 
gum divided into minute portions, and stored up out of the way 
of the nutrient fluid, which would otherwise dissolve it whilst cir- 
culating. In all instances, the stores of this substance appear des- 
tined for the nourishment of young parts ; since they are found in 
the neighbourhood of these, and are exhausted by their growth. 
Thus, starch forms a large part of the substance of all seeds ; 
sometimes (as in the Corn grains) being deposited around the 
germ of the young plant; and in other cases (as in the Pea and 
Bean) being included within it, forming the large fleshy cotyledons 
or seed-leaves, which first come to the surface after the seed has 
begun to sprout, and which wither in proportion as the young 
plant developes itself. Starch is found abundantly, again, in the 
fleshy underground stems destined to nourish young shoots; as 
are the tubers of the Potatoe, and the rhizoma of the Arrow-roots ; 
and it has been lately pointed out that, if the blossoms be pulled 



USES OF THE DEPOSITION OF STARCH. 209 

off the plants before opening, the accumulation of starch will be 
much greater, in consequence of the exhaustion of the store having 
been prevented. Starch is also abundant in the fleshy roots which 
have to furnish nutriment to the young stems, when they first begin 
to grow, as in the Briony and Elecampane. It is also found in the 
pith and bark of many Exogens, and in the cellular tissue occupying 
the centre of the stem of many Endogens (such as the Sago Palm,) 
where it forms a reservoir of nutriment for the young leaves. 

338. The deposite of starch generally continues to increase so 
long as the plant which forms it is in active vegetation. It then 
arrives at its greatest amount, and remains the same until the 
young parts which are to be supplied from it have begun to 
grow ; and then it rapidly diminishes. Thus, it has been stated 
that a hundred pounds of potatoes contain of starch, 

In August lOlbs. In March 171bs. 

September 14|lbs. April 13flbs. 

November 171bs. May lOlbs. 

339. Although this deposition of starch fulfils a part so evi- 
dently important in the vegetable economy, we cannot doubt the 
wise and benevolent intention of the Creator, in thus providing a 
store of nutritious and palatable food for man in situations in which 
he can so easily obtain it ; and it is interesting to remark that, 
from the completely separate form in which it exists, it may be 
obtained in a state of purity from many vegetables, which, as a 
whole, are of very poisonous character. An illustration of this fact 
occurs in the Cassava, which forms a most important article of 
food in almost all the warmer regions of the globe. This substance 
is the starch contained in the root of a plant termed JatropJia 
Manihot; and the root also contains a juice so poisonous, that it 
is employed by some of the savages among whom this plant 
abounds, to tip their arrows and spears. The root is usually 
ground or rasped into a sort of coarse meal; and from this, when 
put under pressure, the juice runs off, leaving the starch nearly 
pure. The Tapioca of Brazil is nearly the same with Cassava. 

340. Starch cannot be applied to the nutrition oi' the tissues 
however, without undergoing an important change, which reduces 



210 CONVERSION OF STARCH INTO SUGAR. 

it, in fact, to the condition of sugar or gum. Of this change there 
are many instances in the progress of vegetation. That which is 
best known is the conversion of the starch of seeds into sugar, 
which takes place during germination ; and upon this the process 
of mailing is founded. The grain of barley contains a large 
quantity of starch ; but, when the embryo is made to sprout, this 
starch is converted into sugar for its nourishment. Now the ger- 
mination of the seed is caused by steeping it in water, and then 
placing it in a warm atmosphere ; and this is the first stage of the 
process of malting. As soon, however, as the growth of the em- 
bryo has proceeded far enough for the proper quantity of the 
starch to be converted into sugar (which is known by the length 
of the young root and by the appearance of the grain itself,) the 
germination is checked by the application of a higher degree of 
heat, which kills the young plant ; and the newly-formed sugar 
can then be employed to give sweetness to water or other fluids. 
In the same manner, the starch of Potatoes and other tubers, is 
converted, when required for the nourishment of the growing 
buds, into sugar, which is absorbed by their vessels ; and nearly 
the same may probably be said of every other instance in which 
starch is laid up for a purpose of this kind. 

341. Now this change of starch into sugar is one of a purely 
chemical nature ; for it can be performed in the laboratory of the 
chemist, by pouring hot water on the starch, so as to break the 
vesicles and set free the contained gum ; and then treating this 
with a weak acid for some time ; by which the whole is converted 
into a sugar that scarcely differs from that of other kinds. In the 
Vegetable economy, however, this change is effected by another 
means. In the juices of the plants themselves, there is a substance 
termed diastase, very minute quantities of which have the re- 
markable property of changing starch into sugar. This diastase 
exists in seeds, and is found in larger quantities near the eyes or 
young buds of the Potatoe, by the vessels of which it is carried 
through the mass of starch when required. How beautiful an ar- 
rangement it is, that a substance possessed of the remarkable pro- 
perty of converting starch into sugar, should be formed wherever 



NUTRITION OF PARASITIC PLANTS. 211 

a store of the first of these substances is laid up for the purpose 
of affording a supply of the latter when required, — and that this 
diastase should be found nowhere else than in the very parts of 
the vegetable structure in which it will be of use ! 

342. We see, then, that the form in which nourishment is 
conveyed to the growing parts of plants is that of gum or sugar. 
These two substances are composed of the same elements in 
nearly the same proportion ; and the former may be changed into 
the latter. They are usually found together in that thick muci- 
laginous* fluid which lies between the bark and wood in summer, 
and which is gradually organized, or converted into tissue ; and 
also in that which forms the pulp of the very young seeds which 
exist in the seed-vessel before the flower has fully expanded. The 
gumminess of this fluid is at once perceived by its glutinous pro- 
perties ; and that it contains sugar is known by the sweetness of 
its taste. Gum and Sugar, therefore, are to be considered as the 
materials out of which the Vegetable tissues are constructed ; and 
Starch must be converted into one of these before it can be ap- 
plied to a similar purpose. 

343. Now the proper juice elaborated by the leaves of one 
plant may sometimes serve for the nourishment of another. A 
group of parasites, which, having leaves of their own, can elabo- 
rate for themselves the crude sap they obtain from the roots of 
another tree, has been already described (§ 320,) but there is 
another which is destitute of leaves as well as of roots, and which 
is therefore dependent for support on the elaborated sap of the 
plants, to which its different kinds respectively attach themselves. 
And as the nature of the proper juice of each species varies much 
more than does that of the crude sap, these parasites cannot sub- 
sist upon the fluids of many different species, but are for the most 
part restricted to those of a few. Most of them grow upon the 
roots or underground stems of others ; no part of them appearing 
above the surface (in general at least) except the flower-stalks 
which are occasionally sent up. They abstract the nutritious 
fluid from the plants to which they cling, by means of a number of 

* Mucilage is the term applied to a solution of gum in water. 



212 LEAFLESS PARASITIC PLANTS. 

little suckers which are formed upon their roots, and which fix 
themselves to the bark of the stems and roots ; and in this man- 
ner, they not unfrequently cause the death of the plant, by draw- 
ing off its juices. 

344. One of the commonest kind is the Orobanche, or Broom- 
rape, so named from the ravages it is thought to commit on the 
Broom and Gorse of our heaths. The different species of this 
plant infest different kinds of vegetables; thus the one which in- 
fests broom and furze also attacks clover ; and in many parts of 
Flanders, the farmers are altogether deterred from the cultivation 
of clover by this species, of which the seeds lie dormant in the 
soil, until it is made to support plants upon which the parasite 
can grow, and which it then attacks vigorously. Another species 
of this genus confines itself to certain Composite flowers, as the 
Centaury and the Scabious ; and this occurs on the red Clover and 
on the Cats-ear ; and one species is found exclusively upon the 
roots of Hemp. The Cuscuta, or Dodder, is another plant of the 
same description, which attaches itself to the stem of the nettle, 
clover and other plants, round which it coils in a direction con- 
trary to that of the sun. When luxuriant, the Dodder gives a 
strange appearance to the herbs and bushes on which it grows, 
covering them, as it were, with a veil of reddish, leafless stalks, 
studded with blossoms. Their seeds, unlike those of most other 
parasitic plants, germinate in ordinary soil ; but if the seedlings be 
kept there, they will soon wither and die, from the want of their 
peculiar nutriment. Some parasitic species derive a part of their 
aliment, in their adult condition, from true roots spread through 
the soil ; but are still dependent for most of the solid matter they 
require, upon the supply of ready-elaborated sap, which they obtain 
by their suckers from the plants upon whose bark they fix them. 

345. From these naturally parasitic plants, we may pass to 
those which are rendered so by artificial means. It will be here- 
after explained (Chap. XII.,) that the cultivated varieties of plants 
cannot be propagated with any certainty by seeds, from which 
we are only sure of obtaining new plants of the same species, (§15.) 
Thus, the seeds of a Golden Pippin or of a Russet, sown in 



PROPAGATION BY BUDS AND LAYERS. 213 

different soils, will all produce plants bearing Apples of some sort; 
but these are not likely to bear any greater resemblance to the 
parent or to each other than all Apple-trees have to their kind ; 
and the character of their fruit will be quite uncertain, — it being 
little better, if the soil be poor, than that of the Crab, from which 
all the varieties of Apple have originated. 

346. In order to propagate any particular variety of fruit or 
flower, the cultivator reserves some of the leafy buds of the tree 
or plant, and places these in circumstances favourable to their 
growth. In many instances, the leaves or leaf-buds have the 
power of forming roots for themselves; and this is especially 
the case when the neighbouring part contains a temporary supply 
of nourishment for them, such as the tuber of the potatoe imparts 
to the eyes, or buds it contains. Thus, if the young branches of a 
Vine be cut into as many pieces as there are leaf-buds, and these 
be properly laid in a favourable soil, and stimulated to growth by 
heat and moisture, they will soon put out roots and become per- 
fect plants ; being at first supported by the nutritious matter con- 
tained in the wood to which they adhere, and afterwards by the 
products of its decay. It is in this way that Sugar-cane is propa- 
gated, — the plants that spring from these cuttings being more 
vigorous, and coming earlier to maturity, than those raised from 
seed. This method is often employed by the Gardener ; who 
sometimes varies it, by not detaching the bud from the parent 
stock, but by bending a branch into the earth, and letting it be 
partly supported by the juices of its parent, until it has put forth 
roots for itself. This is termed propagating by layers. 

347. But there are many cases in which it is desirable not to 
trust to the power which the bud may possess of forming roots 
for itself; and advantage is then taken of the tendency which the 
growing parts of plants have to adhere and become united to each 
other. Such adhesions not unfrequently take place from natural 
causes. Thus, if two branches, either of the same or of different 
trees, be lying across each other, in such a position as to rub against 
one another when moved by the wind, the bark will be worn off 
from each, and a fluid will be exuded from the wounds which 

19 



214 PROPAGATION BY GRAFTING. 

will be in time converted into solid tissue. This is capable of 
conveying sap from one branch to the other; for a tree that has 
been thus united (for the sake of experiment) to two others, and 
has been then cut off from all communication with the soil, has 
continued to live, without any other supply than that which it de- 
rived through these trees. This natural adhesion of vegetable 
tissue is well seen in the ivy ; the branches of which often inter- 
lace and graft together in various places, until the whole forms a 
rude net-work, enclosing the trunk of the tree on which it has 
climbed. 

348. Now the gardener imitates this process, when he wishes 
to supply the separated buds of a tree or plant which he desires to 
propagate rapidly, with nourishment ready to be elaborated by its 
leaves. He chooses a stock, or stem deprived of its own buds, 
and cuts off its top in a sloping direction, so as to expose a large 
surface of wood and bark. He cuts the lower end of the young 
branch, termed the graft, in a similar manner, and then fixes them 
together, taking especial care that the bark and wood of the one 
should meet and join with the bark and wood of the other. If the 
operation succeeds, the stock and the graft become so completely 
united together, as to form in time but one tree, in which all mark 
of the original separation has disappeared. The stock draws up 
from the soil the fluid which the leaves of the graft require; these 
obtain carbon from the air, and elaborate the crude sap into proper 
juice, a portion of which is supplied by the graft to the stock for the 
extension of its own tissues, just as if the stem really belonged to it. 
349. To effect this object, it is generally necessary to choose 
as the stock, a plant either of the same species with the graft, or 
one very closely allied to it ; and the less the relationship, the more 
care and precaution must be taken to secure a union, by bring- 
ing the newest layers of bark and wood into contact. It is cus- 
tomary to select for the purpose some less valuable form of the 
same species ; thus the cultivated varieties of Pears and Apples 
are grafted upon the Wild Pear and Crab. Or a species nearly al- 
lied will sometimes answer almost as well, and, from being readily 
procured, is commonly employed ; thus, Peaches and Apricots are 
grafted on the common Plum. The operation does not always 



PROPAGATION BY GRAFTING. 215 

succeed between two species of different genera ; and it fails en- 
tirely, if an attempt is made to unite individuals of different fami- 
lies. Thus, for example, Pears answer well upon Pears, nearly 
as well upon Quinces, less freely upon Apples or Thorns, and not 
at all upon Plums or Cherries which are of a different family. 
The Lilac will take upon the Ash, notwithstanding their great 
apparent difference, because they are of the same natural family ; 
but the Olive, which also belongs to the same family, cannot be 
profitably grafted upon the Ash, since the vegetation of these is 
too different to allow them to live long together. 

350. From what has been said regarding the readiness of the 
Misseltoe (which may be considered as a natural graft) to grow 
upon various kinds of trees, and the great similarity of the as- 
cending sap in most of these, it is evident that the cause, which 
thus restrains the gardener in the choice of his stock, is not merely 
the difference in the properties of the fluids of the two kinds, but 
also the difference in the general character of their growth. It 
is essential that the stock and graft should be naturally in sap at 
the same time ; and this is more likely to be the case in nearly- 
allied species than mothers. However, in very succulent plants, 
such as the Cacti, of which the fleshy stems are always full of fluid, 
grafts of very different species succeed very well together ; and 
this exception helps to prove the rule. It is necessary, also, that 
the rate of growth of the two should be nearly the same ; for, if 
the graft be of more rapid growth than the stock, and more be 
sent down to the latter than it can convert into tissue, a swelling 
will be formed above the line of union, like that which takes place 
when a cord is bound round a stem (§ 144 ;) and this will increase 
so as in time to cause the death of both parts, by altogether ob- 
structing the passage of fluid. 

351. Not only does the process of grafting enable the gardener 
to multiply with greater rapidity, and to preserve with more cer- 
tainty, any valuable kinds of flower or fruit, but, by the judicious 
selection of a stock, a favourable influence may be produced upon 
them. Thus, the more delicate kinds of Vines produce larger 
and finer grapes when worked upon coarser and more robust 



216 



ERRONEOUS IDEAS RESPECTING GRAFTS. 



kinds ; and the Double Yellow Rose, which seldom opens its flow- 
ers, and will not grow at all in many situations, blossoms abun- 
dantly, and grows freely, when grafted on the common China Rose- 
Some statements, however, which impute to the stock a much 
greater influence, are without any foundation in truth. Thus, it 
has been asserted that Roses became black when grafted on Black 
Currants; and Oranges crimson if grown upon the Pomegranate: — 
but this is altogether erroneous, as these species will not unite at all. 

352. Errors in regard to the success of the progress have 
arisen from an occurrence that sometimes takes place, — the forma- 
tion by the graft of independent roots, which supply it partly or 
wholly with nourishment, with little or no assistance from the stock. 
In this way has been explained the fact that the Olive has been 
made to grow upon the Fig tree (as recorded by Columella, one of 
the earliest writers upon Agriculture ;) for no proper union can 
take place between them, on account of the wide difference in 
their character. Mention is made by Pliny of a tree in the garden 
of Lucullus, which was so grafted as to bear pears, apples, figs, 
plums, olives, almonds, grapes, &c. ; and at the present time the 
gardeners of Italy sell plants of Jasmines, Roses, Honeysuckles, 
&c, all growing together from a stock of Orange, or Myrtle, or 
Pomegranate, on which they say they are grafted. But this is a 
mere cheat ; — the fact being, that the stock has its centre bored 
out, so as to be made into a hollow cylinder, through which the 
stems of Jasmines and other flexible plants are easily made to 
pass, their roots intermingling with those of the stock. After 
growing for a time, the increase in the diameter of the stems thus 
enclosed forces them together, and they assume all the appearances 
of being united. Such plants are of course very short lived. 

353. It maybe useful here briefly to retrace the mode in 
which the elaborated sap is prepared and circulated. The roots 
(or, failing them the general surface of the plant, especially the 
leaves and young bark,) absorb fluid, which consists of water, 
usually having some carbonic acid and ammonia diffused through 
it, and also containing a small proportion of earthy matter, 
(§ 169.) This fluid is conveyed to the leaves, in part by the 
attraction which they have for it, and in part by the propelling 



GENERAL COURSE OF THE SAP. 217 

force of the roots, Q 116.) Whilst ascending the stem, it is 
mixed with some of the fluid previously elaborated, and it under- 
goes some changes, in which oxygen is set free, and in which the 
quantity of gum and sugar contained in it is increased. In the 
leaves, a large quantity of superfluous fluid is parted with, by 
exhalation and simple evaporation; and a great deal of additional 
carbon is obtained by the green surfaces from the carbonic acid 
of the air, under the influence of sun-light ; at the same time a 
small quantity of carbonic acid is being constantly set free from the 
whole surface by the process of respiration, oxygen being ab- 
sorbed. 

354. These are the principal changes which can be detected 
by the observer ; but there must be others of a much more extra- 
ordinary nature, taking place within the vessels of the plant, by 
which, from the simple elements just enumerated, those peculiar 
substances are formed, which are to serve for the nutrition of 
the structure, or are to be laid up for some yet unknown purpose 
in its economy. Of the mode in which water and carbonic acid 
are changed into gum or sugar, the chemist is entirely ignorant ; 
and although these are the most simple of all the extraordinary 
conversions which take place in the assimilation of inorganic 
matter, he is completely unable to imitate it. There is reason to 
hope, however, that he will not long remain so ; since some ani- 
mal compounds have been produced by artificial means. 

355. A still greater mystery is the process by which the 
elaborated sap is converted into cellular tissue or any other form 
of vegetable structure. Some parts of this process have been 
observed, and will now be described ; but of (he cause of the 
changes, nothing is known. The young seed, before the flower 
has expanded, is filled with a sort of sweetish mucilage, which is 
ready to become organized. The first step consists in the appear- 
ance, in what was before a nearly transparent fluid, oi' a large 
number of very minute granules. Soon afterwards larger granules 
appear, round which the smaller ones cluster; and they soon 
present a regular form, resembling that of pieces of money, 
being flattened and circular disks. On one surface oi' each oi' 

19* 



218 FORMATION OF CELLS FROM GUMMY FLUID. 

these, a delicate membrane is seen to project, just as a watch-glass 
projects from the face of a watch ; and this membrane gradually 
extends much beyond the original disk, so as to form a kind of 
bag, in one wall of which that body is included. Still, the mem- 
brane is of so delicate a consistence, that it is easily dissolved 
away by shaking the vessel in which the process is being observed; 
and it is not until some time afterwards that it acquires any con- 
siderable firmness. During the period of the formation of the 
cell, the space between the membrane and the original disk is 
filled with fluid ; and in this a regular circulation may be seen to 
take place, — several currents proceeding from the nucleus (or 
cytoblast as it is technically called) and returning to it again. 

356. When the cell becomes mature, the original disk is ab- 
sorbed, and no farther movement of fluid is seen within the cavity; 
but there are some cells in which it always remains, appearing 
as a dark spot in their walls; and in these the circulation of fluid 
generally continues. This circulation may be well seen in the 

# beaded hairs of the Trarfescantia Virginica, (Vir- 
ginian Spider- wort,) which consist of several dis- 
tinct cells ; at the bottom of each of these, the disk 
or nucleus may be seen, and several currents may 
be observed to proceed from it and return to it 
again. It is a circulation of this kind which has ex- 
Fig. 45. cited much attention in the stem and branches of 
a^oTTrad^ the Char0 ' ( a litt]e cryptogamic aquatic plant) 
cantia. which consist only of large cells laid end to end. 

The fluid passes down one side of the stem and up the other, 
turning round at each extremity. If the stem (which is usually 
composed of a single cell, sometimes many inches long) have a 
thread tied round its centre, so as to separate the original cell 
into two, each of these will have a complete circulation of its own. 
A similar movement of fluid has been seen in the Frog-bit (ano- 
ther aquatic plant of this country) and in many others ; and it is 
nearly certain that it takes place in every vegetable cell that ex- 
ists, during some period of its growth ; being only visible for a 
short time in some, which soon arrive at a condition little subject 



NUTRITION IN CELLULAR PLANTS. 219 

to change; but continuing during the greater part or the whole 
of life in others. 

357. This movement of fluid in the individual cells, is quite 
distinct from the general circulation which has been described in 
the higher plants. It is a part of the process of formation, by 
which the nutritious fluid that is brought to each part is converted, 
into organized tissue. In the simple Cellular plants, where the 
same surface performs alike the functions of absorption, exhala- 
tion, digestion, and respiration, there is no general circulation of 
fluid; since each of the cells composing the whole structure 
imbibes the materials of its nutriment for itself, and converts 
them into the substance of its own tissue, or employs them in the 
production of new cells. These seem to be usually developed 
from the fluid within the parent, in the same manner as the cells 
of the young seed are produced from the gummy matter it con- 
tains, as just now described ; and the analogy is the more close, 
since the membrane lining the seed may be regarded as itself a 
single large vesicle. The increase in size of any organ is occa- 
sioned in part by the enlargement of each individual cell, and 
in part by the development of new ones, which are formed in 
some instances between those previously existing, and in other 
cases (especially in the root) at the extremity only. 

358. In the simplest Cellular plants, therefore, there is no 
necessity for any general circulation of fluid ; and no other move- 
ment is seen but that which occurs in single cells. But in the 
more highly-organized tribes, where the parts which receive the 
different kinds of food from the elements around are at a distance 
from each other, and from those to which the nutritious fluid 
must be supplied, a general circulation is required to bring them 
all into connexion ; and this is accordingly found to exist, so that 
every part of the structure is nourished by a fluid that has been 
elaborated by a system of organs, of which each is particularly 
adapted to a single object, whilst the actions of all arc directed to 
a common purpose. This elaborated sap, being supplied to the 
growing parts of a plant, gives to them all the means of develop- 
ment that they can require; and they then only need the influence 
of light and heat, to perform their respective actions with vigour. 



CHAPTER X. 



QF THE SECRETIONS OF PLANTS. 



359. We have seen that the elaborated sap contains the 
materials of the various tissues of the vegetable fabric ; and an 
outline has just been given of what is known of the mode in 
which they are converted into living structure. The principal 
uses to man of the various kinds of these structures, will be best 
stated when the chief groups of plants are described, in the second 
division of this volume. We have next to consider a class of 
products, which are not of the same character; for they serve no 
obvious purpose in the nutrition of the plant itself, and are never 
converted (so far as can be ascertained) into the materials of its 
tissues. They usually make their appearance in the elaborated 
sap; but not unfrequently they are afterwards separated in some 
degree from it, and stored up (as it were) in a particular portion 
of the plant. In Animals we find a provision of a similar kind. 
The blood not only contains the elements of the solid tissues which 
are to be nourished by it, but also of fluid secretions, which are 
separated from it by special organs. Hence the term secretion, 
which means a separation, or setting-apart, is derived. 

360. In Animals, however, such secretions are usually destined 
to answer some obvious purpose, either in the system or out of it. 
Thus the secretion of saliva serves to moisten the food, and that 
of gastric juice to digest it ; and in this process it is one function 
of the bile to assist. Again, the secretion of milk in the female 
for the nourishment of the young, that of poison in the venomous 
serpent for the destruction of its prey, that of the glutinous fluid 
with which the spider constructs its web, are instances of the 



GENERAL CHARACTERS OF VEGETABLE SECRETIONS. 221 

separation of certain ingredients of the blood, which are sent out 
of the body for particular objects. But secretion in animals has 
other purposes; — namely to purify the blood from certain ingre- 
dients, which, if they accumulated in it, would occasion disease 
and even death. This is the purpose of the separation of car- 
bonic acid by the lungs ; and also, in part, of the secretion of 
bile, which carries off a large quantity of the superfluous carbon 
of the system. In the same manner, the secretion of urine carries 
out the superfluous nitrogen (which exists very largely in this 
fluid, § 195, 197.) 

361. Now in regard to the secretions of plants, it is very re- 
markable that, whilst in number and variety they much exceed 
those of animals, the use of them in the Vegetable economy should 
be much more obscure. In a few instances only are they destined 
to be sent out of the system ; they are usually deposited in some 
part of it ; yet they are not even separated in every instance from 
the nutritious part of the juices in which they are at first formed. 
The Secretions of plants comprehend all the peculiar products 
which do not form part of their tissues; thus, all the vegetable 
dyes, the active medical principles, the oils, the resins, &c, and 
the aromatic or volatile oils, belong to this class of products. Now 
as the substance of which the tissues of plants are composed is 
every where almost the same, any varieties which these tissues 
may present, in colour, taste, &c., must be due to them ; and it is 
from their presence that each plant derives its particular charac- 
ter, either as an article of food, or as furnishing products useful 
in medicine or the arts. The pure vegetable tissue, and the nu- 
tritious gum or starch combined with it, are nearly tasteless; and 
the alburnum or sap-wood of trees possesses neither toughness 
nor colour. The former may be rendered uneatable by the disa- 
greeable taste or injurious nature of the secretions diffused through 
it; the latter is strengthened, and receives its peculiar colour, by 
the deposition in its cells and tubes, of products which have been 
separated from the circulating fluid, and which give to the wood 
a density proportionate to their amount, and to their own power 
of subsequently hardening. 



222 THEIR DEPENDENCE UPON LIGHT. 

362. The formation of these secretions is still more dependent 
on the influence of light, than is that of the nutritive materials 
themselves. Many plants, which, under the rays of a tropical sun, 
produce secretions of a powerful character, whether as medicines, 
as aromatics, or as dyes, are almost inert in colder climates, even 
when the amount of heat artificially given may fully equal that to 
which they have been accustomed. Thus, the Tobacco of Persia 
is universally celebrated for its peculiar perfume ; and from the 
Roses of the South alone is it worth while attempting to obtain the 
powerful essential oil, which is known as Otto or Atar of Roses. 
This principle is advantageously employed in the growth of vege- 
tables for the table ; for, if they are reared under a diminished 
light,, many kinds of plants may be used as food, which naturally 
contain secretions either unpleasant in taste or injurious in cha- 
racter. Such are the Sea-kale, Lettuce, and Cichory ; which are 
prevented from becoming rank, by heaping earth around their 
young shoots, or by growing the entire plant in a dark situation. 
The peculiar secretions, too, are not present in young plants, all 
whose energy seems expended in the extension of their own struc- 
ture ; hence those kinds which are afterwards rank poisons, may 
be eaten with impunity at an early period. Thus the peasants of 
Languedoc employ yr-ung poppies as food ; and cattle do not reject 
noxious weeds in spring, which their instinct would not permit 
them to touch in summer. 

363. As the special secretions of plants are formed in the ela- 
borated sap, they will not be found in those parts to which it is 
not afterwards conveyed. They may generally be traced first in the 
leaves; but in the course of their descent, they are often separated 
by some particular organ, in which they are concentrated (as it 
were) to the exclusion of the rest. Thus many of the most power- 
ful medicinal agents are obtained from the bark ; some abound 
most in the roots; other products, especially resins and colouring 
substances, seem to be chiefly deposited in the wood ; fixed oils 
are generally conveyed to the seeds, where they seem to be depo- 
sited for the same purpose as starch, — the nourishment of the em- 
bryo; whilst aromatic oils are generally found either in the leaves, 



SPECIAL SECRETIONS. TANNIN. 223 

the leafy parts of the flower, or in the coats of the seed or fruit. 
Not unfrequently certain little bodies, which have received the 
name of glands, are seen on the surface of the leaves, from which 
fluids are poured forth for various purposes. Thus the Nettle is 
covered with glands of this kind, that secrete an acrid fluid, which, 
being conveyed through a pointed tubular hair mounted upon the 
gland, produces an irritation in the wound made by the hair, just 
as does the poison of the tooth of the serpent or the sting of an 
insect. The little Dor sera (Sundew) again, exudes a gluey secre- 
tion from the surface of its leaves, which serves to attract and re- 
tain Insects, the decay of whose bodies seem to contribute to its 
healthy existence, as it does to that of the Dionsea (§ 246.) 

364. A detailed account of the various secretions of plants 
would not be adapted to this work ; and we shall confine ourselves 
here to a notice of those which are most serviceable to mankind. 
Of all these, there is perhaps none more directly important than 
that denominated tannin; although of its use in the economy of 
the plants that produce it nothing is known. Tannin is the sub- 
stance, by the chemical agency of which upon animal tissues con- 
taining gelatin (the material commonly known as glue, which 
forms a large part of the skin of most animals,) leather is produced. 
Its chemical effect upon gelatin may be shown by steeping some 
oak-bark, or bruised gall-nuts, in water ; and then adding some 
of this fluid to water in which glue has been dissolved. A quantity 
of flaky matter will fall down, which is, in fact, leather; — its par- 
ticles being separate from each other, on account of the liquid 
form in which the elements were brought together. The process 
of tanning consists in steeping the skins to be converted into 
leather in a solution of tannin ; this slowly penetrates their sub- 
stance, converting their gelatin, which would otherwise soon un- 
dergo putrefaction, into the compound just mentioned, which is 
capable of resisting decay. And, as no injury to the texture of 
the skin is done by this process, it is converted into a substance, 
which from its pliancy combined with toughness, and durability, 
is useful for a great variety of purposes. 

365. In this country tannin is principally obtained from oak- 



224 SOURCES OF TANNIN. 

bark ; but as of late years the supply of that material has not been 
equal to the demand, it has been necessary to look for some other 
source from which it may be procured. Several other trees com- 
mon in this country yield tannin ; such are the elm, willow, elder, 
plum, sycamore, birch, cherry, poplar, hazel, and ash ; — but the 
proportion contained in all, save the first two of these, is not suf- 
ficient to render their cultivation for this object a source of profit. 
Even the common heath has been applied to this purpose ; the 
tannin being extracted from it by boiling. There are, however, 
many trees of tropical climates, which contain a larger proportion 
of tannin than that yielded by oak-bark. One of these is a kind 
of Sensitive-plant (Acacia Catechu,) which flourishes abundantly 
in the mountainous parts of Hindostan, and yields the substance 
known as Catechu, or Terra Japonica (Japanese earth, from its 
earthy appearance,) which is much valued in medicine from its 
astringent properties, and which acts very powerfully on gelatin. 
It is a dry extract, prepared by boiling the heart-wood of the tree, 
cut into chips, and then evaporating the superfluous water. 

366. The Mangrove tree, of the East and West Indies, is ano- 
ther form which a large quantity of tanning matter may be ob- 
tained. This curioustree growson the borders ofthesea and on the 
banks of rivers ; its stem is supported by a large number of branch- 
ing roots, which rise out of the water in arches several feet high, 
closely intertwining with each other; and the branches hang down 
and send forth similar roots, as in the Banyan (§ 152.) The ex- 
tract made from its bark is used for tanning in many parts of the 
West Indies and in Hindostan ; and it is said to perform its office 
more perfectly in six weeks than oak-bark does in ten, producing 
a leather more firm and durable. In New Holland there is an 
abundance of a species of Acacia, which is cut down for the pur- 
pose of clearing land ; and from this it has been ascertained that 
an extract may be made, fully equal to oak-bark. As, notwith- 
standing the distance of the colony, it can be supplied very cheap- 
ly, so long as there is a superfluity of the Acacias, it will proba- 
bly take the place in great degree of oak-bark. 

367. Another secretion of great importance in the arts, and of 



CAOUTCHOUC. — SOURCES AND PROPERTIES. 225 

which new and valuable applications are constantly being dis- 
covered, is Caoutchouc, commonly known as Indian Rubber. It 
was first brought as a great curiosity from South America about 
150 years ago; and for a long time nothing was known of the 
source from which it was obtained ; nor was it applied to any use- 
ful purposes, except the rubbing-out of pencil marks, from which 
it took its name. It is known to be contained abundantly in the 
juices of many trees growing in tropical climates, as well as, in 
smaller quantity, in many plants of temperate regions; it seems to 
form an essential part of the milky juices (as they are termed, from 
their white colour, rather than from their properties,) which are 
characteristic of several tribes of Vegetables, especially of the 
Artocarpeae (Bread-fruit tribe,) Jjpocynese (Oleander tribe,) and 
Uuphorbiacese (Spurge tribe) which will be hereafter more par- 
ticularly described. To the first of these orders belongs the cele- 
brated Palo de Vacca or Cow tree of South America, which yields 
a copious supply of a rich, bland, and wholesome fluid closely re- 
sembling milk. In the plants of the second order, the milk is 
usually rendered bitter and poisonous by the admixture of other 
secretions ; and in the third it is of a very acrid character. In other 
orders of plants having milky juices, however, caoutchouc forms 
but a very small proportion of them; such are the Papaveracex 
(Poppy tribe) and Cichoracese, (Cichory tribe ;) and here it is re- 
placed by opium, — a substance presently to be adverted to. The 
juices which contain caoutchouc are obtained by making incisions 
into the bark ; and the fluid which runs from them soon thickens, 
on exposure to the air, into a substance of a pure white colour, 
having neither taste nor smell. The dark colour which caout- 
chouc usually presents, is received from the smoke of the fire over 
which it is dried. 

368. The use of Caoutchouc in the arts and manufacture 
results from two distinct properties; — its high degree of elasticity; 
— and its complete impenetrability to water. The modes in which 
its elasticity is made useful are extremely numerous; amongst 
others may be mentioned, the employment of it to form elastic 
webs, which are partly woven with threads spun from it, and which 
20 



226 



USES OF CAOUTCHOUC. 



are introduced into braces, saddle-girths, and other bands in which 
a steady and equable pressure is required. Its impenetrability to 
fluid has long been known, and was applied by the Indians of 
South America in the production of waterproof boots ; these were 
made by spreading the juice, when flowing fresh from the tree, 
over moulds of clay, which could be afterwards broken away from 
their interior. Similar articles have been made in this country by 
keeping the juice in bottles from which the air was excluded ; by 
which means it has been brought over in a perfectly fluid state, 
without losing its power of hardening when exposed to the atmo- 
sphere. 

369. But of late years, a much more effectual and ready 
means has presented itself of thus employing to great advantage 
the valuable properties of Caoutchouc, in the discovery of the 
power of ether and naphtha* to dissolve it without changing its 
properties ; so that a kind of varnish may be thus formed, from 
which, when it is spread over any surface, the dissolving fluid 
(which is extremely volatile) will evaporate, leaving a very thin 
coating of caoutchouc behind. It is in this manner that the water- 
proof fabrics are made, which are now so much employed for 
cloaks, wrappers, &c. ; and as these are also air-tight, they may be 
used for air-cushions, mattrasses, &c. The fabric consists of two 
layers of cloth, which are varnished, each on one side, and then 
passed through a pair of rollers with the varnished sides in contact; 
so that a thin layer of caoutchouc exists between them. Some idea 
of the great and increasing consumption of this substance, new and 
useful applications of which are constantly being discovered,! 
may be formed from the fact that, whilst in 1830 the quantity 
imported into England was more than 52,000 lbs. (nearly 

* This fluid is obtained in England from tiie tar which passes over with 
the gas now so universally employed, when coal is heated in closed retorts. 

f A patent has lately been obtained for the employment of solid Caout- 
chouc in saddles and horse-collars. Two objects are here attained by it; — 
the much increased comfort of the horse by the equal diffusion of pressure 
over the surface, by which galling is prevented ;— and the preservation of 
the padding beneath, by protecting it from being saturated (as it otherwise 
frequently is) with the perspiration of the animal. 



FIXED OILS. OLIVE OIL. 227 

double that imported in the preceding year,) the consumption in 
the year 1833 was nearly 180,000 lbs. ; and there can be little 
doubt that it has since increased in nearly, if not quite, as rapid 
a proportion. 

370. The large number of oils obtainable from plants, may be 
divided into the fixed or fat oils from which no vapour passes off 
at the temperature of boiling water; and the volatile or essential 
oils which give off vapour at or below that temperature. The 
latter are the sources of all the odours diffused so widely through 
the vegetable kingdom ; and furnish, also, some materials of great 
importance in the arts of life. The fixed oils are all obtained by 
pressure from the fruits or seeds of plants, especially those of the 
Nut kind, all of which contain it in greater or less proportion. 
That in the greatest request is olive oil ; which is obtained both 
from the pulpy part of the fruit, and from the seeds ; that drawn 
from the former source is regarded as the best, being less liable 
than the other to become rancid. The olive tree was originally a 
native of Syria, Persia, and other hot countries in Asia ; but it 
has gradually extended itself over the South of Europe and the 
North of Africa. The cultivation of it has been principally 
attended to in times of peace, of which it was considered as the 
symbol. It is extremely profitable to the grower, if properly 
attended to. The young olive plant bears at two years old ; and 
in six years begins to repay the expense of cultivation, even if the 
ground beneath it be not made to yield any other crop. It- con- 
tinues to be profitable for a long period, rivalling the oak in 
longevity, and bearing good crops when the trunk is reduced to 
a mere shell ; so that it is a common proverb where it is culti- 
vated, — " If you want to leave a lasting inheritance to your chil- 
dren's children, plant an olive." Olive oil is very extensively used 
in the south of Europe, in the preparation of various dishes for the 
table, for consumption in lamps, for the manufacture of the supe- 
rior kinds of soap, and for various other purposes. It is used 
very largely in this country, in spite of a heavy duty ; upwards of 
four millions of gallons having been imported in 1S31, of which 
about half was exported again to other countries. 



228 RAPE, LINSEED, HEMP, POPPY, AND NUT OILS. 

371. Rape oil is obtained from the seeds of a species of 
Brassica, a plant closely allied to the Cabbage, which is cultivated 
for that purpose in France and some parts of England. It is 
much used for burning in lamps; and has the advantage over 
others, that it remains fluid at a lower temperature. Linseed oil, 
which is obtained from the seeds of the Flax-plant, is of very 
general application in the arts ; especially in oil-painting and the 
composition of varnishes, for which it is particularly adapted by 
its property of drying on exposure to the air. This power may 
be increased by boiling the oil, which is then termed drying-oil; 
it is in this manner that printers'-ink, which is a sort of paint 
composed of oil and lampblack, is made to dry rapidly. The seed 
of the Hemp-plant yields an oil nearly as valuable ; and it has 
lately been found that a large quantity may be extracted from the 
cotton-seed ; so that each of these three plants is valuable to man 
in two very different ways. The hard cake left after these oils 
have been pressed out from the seed is used for feeding cattle. 
Sun-flower and mustard seeds, also, yield a good oil, which is 
employed in the countries in which these plants abound as a sub- 
stitute for other seed oils. A large quantity of oil is now obtained 
on the Continent of Europe, from the seeds of the Poppy. It 
was commonly supposed, when this oil was first introduced into 
use, that it must partake of the narcotic properties of the plant ; 
but this was erroneous, for oil, like starch, may often be separated 
from the peculiar juices of the plant, without being influenced in 
the slightest degree by their properties. Poppy oil is a very useful 
one in the composition of varnishes, on account of its freedom 
from colour, and its drying quality; in the northern parts of 
France it is much used by soap-boilers. Oil is also obtained in 
many parts of the Continent, especially Switzerland, from Wal- 
nuts and Hazel-nuts ; it is much esteemed by varnishers for the 
same properties as Poppy Oil. The influence of climate on the 
production of oil, is well shown by the fact that, from these nuts, 
which in England would scarcely yield enough to repay the labour 
of extracting, half their weight of oil may be extracted in the 
South of Europe. Nearly the same may be said of the Beech, 



PALM, COCOA-NUT, AND BEN OILS. 229 

from the kernels of which about 27 per cent, of oil is obtained in 
some parts of France and Germany. 

372. Another important vegetable oil is that known under 
the name of Palm Oil ; it is obtained from the fruit of two species 
of Palm which grow in several parts of Africa, especially in Sene- 
gal. One of these is named Cocos butyracea, from the buttery 
nature of its oil, which is much employed by the natives along 
the Gold Coast as an article of diet, and which, when fresh, is 
delicate and wholesome. It is imported into Britain in large quan- 
tities, chiefly for the soap-maker and perfumer. The quantity 
retained for home consumption in 1839, was 276,000 hundred- 
weight. The oil is contained in the kernels of the nuts, which 
are not very different from those commonly known as Cocoa-nuts ; 
these last, also, yield a large quantity of oil, which congeals, at 
the ordinary temperature of the air, into a white fatty substance. 
In Ceylon, where this fruit is most abundantly produced, its oil 
is employed by the natives for a great variety of purposes. It 
makes a most excellent lamp-oil, except from its tendency to 
congeal by a slight amount of cold ; and for this purpose it is 
employed by the Cingalese, whose greatest consumption of it, 
however, is for the anointing their bodies. In this country a 
process has been discovered, by which the oil may be separated 
into two parts ; one resembling fat, which may be applied to the 
making of candles ; whilst the other is as fluid as most other 
oils, and is particularly adapted for lamps. The oil is also well 
adapted to the wants of the soap-maker. Its consumption in 
Britain is much increasing; in the year 1839, nearly 40,00U 
hundred-weight of the oil was employed in various ways in this 
country, and upwards of four million pounds of the nuts were 
imported. The oil known as Ben-oil is of more importance than 
might be supposed from the small quantity of it introduced int< 
this country. It is produced from a tree, growing in the Eas: 
Indies, Egypt, and the Levant, which belongs to the same groin 
with the Tamarind. The peculiarity of this oil consists in its 
very slight tendency to become rancid, and its perfect freedom 
from smell; on which account it is much used by the perfumers. 

20* 



230 ESSENTIAL OR VOLATILE OILS. 

to retain the scent of the more fragrant oils. At a low tempera- 
ture it separates into two parts, the one solid and the other liquid ; 
and the latter is employed by watchmakers, in preference to any 
other oil, for lubricating their delicate works, on account of its 
having no action upon the metals. 

373. The essential or volatile oils are mostly obtained from 
the leaves or flowers of plants ; sometimes, however, they exist in 
the wood and bark, or in seeds. In all instances they possess a 
powerful scent ; and the degree of this depends upon the tendency 
which the fragrant oil has to diffuse itself. In some of the most 
odorous flowers this tendency is so great that the oil cannot be 
procured in a separate form ; and their perfume is obtained by 
causing the flowers to impart it to some fixed oil ; for which pur- 
pose Ben-oil is preferred where it can be obtained. The volatile 
oils are not easily obtained by pressure ; but are readily driven off 
by heat ; but this must not be so great for the most diffusible as 
that of boiling water. To communicate the fragrance of flowers 
to a fixed oil, cotton soaked in it is placed in alternate layers with 
the flowers whose scent is to be obtained, so as to fill a close ves- 
sel, which is then placed in hot water for twenty-four hours ; du- 
ring this time, the fixed oil will have imbibed the rich perfume of 
the flowers, and it is then separated from the cotton by pressure. 

374. The essential oils which are somewhat less volatile may 
be obtained by distillation, in the same manner as spirits. The 
substances which yield them are put into a vessel, with water to 
prevent their being over-heated. When the water is boiled, the 
oil passes away with the steam; and, when both are condensed, 
it floats upon the surface of the water. A large number of oils 
possessing great fragrance and strong taste may thus be obtained 
from different kinds of plants ; and these oils are used in per- 
fumery, in confectionary, and in medicine. The oils of Roses, 
Lavender, Chamomile, &c. are distilled from the flowers; those 
of the various plants of the Mint kind — Peppermint, Spearmint, 
Pennyroyal, &c, from the leaves and stems, which contain a num- 
ber of little receptacles near their surface; that of Sassafras from 
the wood ; that of Cinnamon from the bark ; that of Carraway, 



VOLATILE OILS. TURPENTINE. 231 

Anise, Fennel, and other Umbelliferous plants, from the coats of 
their seeds, in which they are stored up in little receptacles ; that 
of Lemons from similar little receptacles in the rind of the fruit ; 
and that of Nutmeg from the seed itself. Many of these oils con- 
tain Camphor, which may be separated from them by exposure 
to cold. Sometimes the secretion of volatile oil is so abundant, 
as to make itself perceptible in the atmosphere around, to other 
senses besides smell. Thus the Fraxinella gives off so much from 
its leaves, that the air in its neighbourhood is highly inflammable 
in warm weather. There are some substances which seem to 
contain the materials of an essential oil, rather than the oil itself. 
Thus when water is added to flower of Mustard, an acrid and vo- 
latile oil is produced, very irritating to the eyes ; yet no evidence 
of its existence can be obtained without the addition of water, so 
that the latter probably occasions some change of composition by 
which the oil is produced. The volatile oil of Bitter Almonds seems 
to be produced in a similar manner. Perhaps the increased fra- 
grance of our gardens after a shower of rain is due to a similar 
cause. 

375. Into the particular uses of the foregoing oils, this would 
not be the place to enter ; some of them will be noticed in the 
description of the several orders to which the plants that yield 
them belong. There are other volatile oils of much more im- 
portance in the arts and manufactures, which must next be noticed. 
One of the best-known of these is Oil of turpentine, (commonly 
termed Spirit of turpentine) which exists in combination with 
resin, forming what is commonly known as Turpentine, in all the 
trees of the Pine and Fir tribe, as well as in some others. The 
Turpentine is usually contained in special receptacles in the sub- 
stance of the wood ; but sometimes it collects in blisters under- 
neath the bark, which appear during the strong heats of summer. 
It flows from these as a limpid juice, which thickens on exposure 
to the atmosphere, when incisions are made into the stem. The 
common Turpentine is obtained from the Scotch Fir, when 
growing in the South of Europe, and the Southern part of North 
America; but it cannot bo procured in any large quantity from 
the same tree when growing in Great Britain. Superior 



232 TURPENTINE, RESIN, TAR, PITCH. 

kinds are drawn from the Pistacia of Scio, and from the Larch in 
Southern Europe. Turpentine is not itself applied to any impor- 
tant use ; but the two substances which it contains, — the volatile 
oil and the resin, — both serve many purposes. They are separated 
by distilling the Turpentine with water ; which causes the vola- 
tile oil to pass over, leaving the resin behind. Oil of Turpentine 
is extremely useful from its power of dissolving resins, which form 
the basis of most varnishes ; and from its great volatility, it quickly 
flies off or dries away, leaving a thin coat of the varnishing sub- 
stance fixed to the surface on which it has been applied. The 
most extensive use, however, to which it is put, is that of diluting 
oil colours, so that they will flow freely from the painter's brush. 
No other known fluid would answer this purpose ; for it is the 
only one which will mix readily with the paint, (diluting its thick 
oil, as water would dilute a sirup or gummy fluid,) without in 
the least degree affecting its essential properties, — and which will 
also dry rapidly. 

376. The very important substances known as far and pitch 
are also obtained from trees of the same kind ; and they may in 
fact be regarded as impure turpentines, altered by the heat em- 
ployed to separate them. A sort of kiln is built up of billets of 
wood ; and round the bottom of this is a channel for drawing off 
the fluid which runs down whilst the wood is being burned. Tar ' 
may be made from trees which no longer yield turpentine, and 
also from those which have partially decayed on the ground. Pitch 
is tar deprived of its more volatile part ; this may be separated 
either by distilling off the oil, which is an inferior oil of turpen- 
tine, or by burning it; in the last process, the volatile oil, being 
the most readily set on fire, is burned away, and the resinous 
part remains. In this manner, two barrels of tar will produce 
one of pitch; and besides the oil, an acrid passes off, by the dis- 
tillation of tar, which much resembles that obtained during the 
burning of wood from charcoal, and hereafter to be mentioned 
under the name of the pyroligneous. 

377. Several other Resins are yielded by plants; some of 
which commonly termed Gums, are of service in various arts. 



COPAL, MASTIC, BENZOIN, &C. 233 

Such is Copal, which is obtained from a species of Sumach ; but 
though the tree will readily grow in North America and in Eng- 
land, it requires the heat of a tropical climate to perfect its juice; 
and most of this product comes from Africa. Copal is much 
valued as a varnish, on account of its hardness and transparency; 
which qualities cause it to be employed for pictures, fine wood- 
work, and other similar purposes. Mastic is another resin, which 
is used for similar purposes, and is obtained from a tree termed 
the Lentisk, nearly similar to that which yields the Chian turpen- 
tine. Incisions are made in the trunk and branches during the 
hottest parts of the summer ; and the liquid juice which flows from 
them, thickens almost immediately that it is exposed to the air, 
into little drops or tears. Dragons' Blood, so named from its red 
calour, is a resin which exudes in drops from the stem of several 
trees growing in the tropical parts of Asia, Africa, and America ; 
it is valued on account of the tinge which it imparts to spirit of 
wine, and is employed, when thus dissolved, in staining marble 
and woods. The substance called Benjamin or Gum Benzoin, 
is also a resin secreted by a tree that grows in the tropical parts of 
Asia, especially in Siam and Sumatra. This tree grows very rapidly, 
so that it yields resinous juice when only six years old, its trunk 
being then about seven or eight inches in diameter. This resin 
has a very fragrant odour, which probably depends upon its 
having, mixed with it, a small quantity of essential oil. It is 
principally used in perfumery, and in the manufacture of pastil- 
les, or incense, which, when burned, diffuse an agreeable odour. 
Hence the principal consumption of it is in the churches of Ro- 
man Catholic and Mohammedan countries ; and a much larger 
proportion of that brought to London is again exported, than is 
retained in this country. There are many other kinds of resin, of 
which small quantities are employed for particular purposes; but 
the foregoing are those most valuable to mankind. 

378. Resinous matter, however, exists in other products which 
are termed Gum-Resins, from the quantity of Gum they contain; 
and this enables them to be partly soluble in water, which pure 
resins are not in the slightest degree. Some of these are valued on 



234 GUM-RESINS ; MYRRH, GAMBOGE, &C. 

account of their fragrancy ; and have been employed in the in- 
cense burned in places of religious worship, from very early times. 
Thus we find in the earliest records, that the addition of fragrant 
odours was regarded as rendering the sacrifices offered to the 
Deity more acceptable ; and the same idea seems to prevail in 
many Christian as well as heathen countries at the present time. 
Frankincense is one of these substances ; it is produced from a 
kind of Juniper growing in Arabia. Olibanum is another of simi- 
lar character, also produced by a species of Juniper ; and Myrrh is 
nearly allied to these, but the source of it is uncertain. Gamboge is 
a gum-resin of very different properties, which is the product of 
several different kinds of trees growing in Ceylon, Siam, and Cochin 
China. It flows out in a liquid form when incisions are made in 
the bark, and is afterwards made solid by the heat of the sun ; but 
it also occasionally exudes from the surface in tears. When rub- 
bed with water, it forms a bright yellow fluid, which is much 
employed in water-colour drawing ; the water dissolves the gum, 
whilst the resin remains suspended in the form of very minute 
particles, which may be seen with the microscope. The whole is 
dissolved in spirit of wine ; and this solution is used as a laquer, 
to heighten the colour of brass- work, by its golden hue. Gamboge 
is also a powerful medicine, having a violent purgative effect ; and 
with aloes it is the principal active ingredient in the nostrum 
known under the name of Morison's Pills* 

379. The true Gums may next be noticed ; these are dis- 
tinguished from the previous substances, by their being entirely 
soluble in water, whilst spirit of wine does not act upon them. 
Their solution in water is a thick adhesive fluid, which is used for 
many purposes in the arts. It serves to unite substances together 
in the same manner as glue ; and may be used in cases where heat 
is undesirable. Its chief employment, however, is in calico-printing, 

* It is probably to the bad mixture of the ingredients, by which an 
undue proportion of this active substance has been contained in a particu- 
lar batch of pills, that some of the deaths which have occurred from their 
use, are to be attributed ; and in other instances they haye resulted from 
the enormous number of pills taken. 



GUM ARABIC ; GUM ACACIA. 235 

being used to stiffen the cloth before the colours are applied, 
so that they are prevented from running into each other and 
becoming indistinct. As all trees contain gum in their sap 
(§ 328.,) it might be obtained in some degree from any ; but it 
naturally exudes in large amount from some kinds, which, there- 
fore yield it most readily when incisions are made in the bark. 
The kind of gum termed Gum Arabic, which is the one most 
valued, is obtained from a species of Acacia, which flourishes in 
almost every part of Arabia and Middle Africa ; but it is only in 
the hottest regions, that the gum is produced in much abundance. 
When the tree first opens its flowers, the gum begins to exude 
spontaneously from the bark of the trunk and branches ; in the 
same manner as it is often seen to do from the cherry-tree in this 
country. In the spring, however, when the weather is very dry, 
the gum can only be obtained by incisions made in the bark. 

380. Gum Senegal is similar to Gum Arabic, being obtained 
from a kind of Acacia differing very little from that which yields 
the latter ; but it is of inferior quality. Gum Tragacanth, which, 
is obtained from a low prickly shrub growing in the Levant, is in 
some respects different from the foregoing. It does not dissolve 
freely in water ; but forms a thick mucilage with a certain definite 
proportion of it. If this be mixed with a larger quantity of 
water, it will separate again after a time and fall to the bottom, 
leaving very little gum in the water above. This gum is employed 
in some kinds of calico-printing, in which the chemical action of 
the dyes on the other gums would injure their qualities. The 
plant which yields this gum would flourish in England ; but it 
can here only prepare enough of it for its own support, and only 
possesses a superfluity under the influence of a warmer climate. 
A large quantity of a gum resembling that of the Acacia, may be 
obtained from the various species of lichen growing in this country, 
by the action of hot water. The similarity of starch to gum has 
been already noticed, and some of the sources from which it may 
be obtained have been pointed out ; and it is here, therefore, only 
desirable to add, that the gum obtained from starch is much used 
in the arts, especially for the purpose of stiffening different 



236 WAX PRODUCED FROM VEGETABLES. 

fabrics, on which account it is employed largely by calico-print- 
ers, under the name of British Gum. 

381. The next Vegetable secretion to be noticed is Wax; 
which, though commonly supposed to be formed by the Bee alone, 
is undoubtedly present in many plants also. It may be seen in 
the form of minute scales upon the outer surface of the Plum and 
other stone fruits, forming what is known as the bloom; and it is 
by the existence of a thin coating of it, that the leaves of the Cab- 
bage, Tropseolum (Sturtion,) and other plants are enabled to re- 
sist moisture. Wax may be obtained by heat, though in small 
quantity, from the poplar, alder, and several other plants ; but it 
exists in such abundance in the fruit of a Virginian myrtle, that 
it has received the name of candle-berry. In the parts of the 
country where this tree abounds, it is quite worth while to collect 
the berries for the wax they yield, which, when made into can- 
dles, burns with peculiar brightness and freedom from smoke, at 
the same time giving off a fragrant odour. Another wax-bear- 
ing tree exists in South Africa ; and the substance yielded by its 
berries, which is made into candles by the Dutch, is greedily eaten 
by the Hottentots. In South America there is a kind of palm, the 
leaves of which have their surface covered with minute scales of 
wax, which separate when they are dried in the shade ; and of 
this wax, mixed with a small proportion of tallow to avoid brit- 
tleness, excellent candles may be made. The leaves are so little 
permeable to moisture, that they may be used as coverings for 
houses ; and they have been known to sustain the vicissitudes of 
weather for twenty years in such situations, without requiring to 
be renewed. The pith and the fruit of this palm also furnish a 
nutritious food for man and cattle ; and the wood is useful in 
building houses, making fences, &c. ; so that it is a very important 
tree in the district in which it abounds. Another species of Wax 
Palm is found in the more elevated parts of South America ; 
growing on the mountain ranges, to the prodigious height of a 
hundred and sixty feet. The wax here exists in the form of a 
kind of varnish covering the trunk. 

382. A substance nearly resembling Tallow is yielded by a 



VEGETABLE TALLOW. CAMPHOR. 237 

tree named the Croton sebiferum which grows abundantly in 
China, and is described as being the largest, the most useful and 
the most widely diffused, of any of the plants of that country. It 
imitates the oak in the height of the stem and the spread of its 
branches. The seed-vessels are hard brownish husks, not unlike 
those of chestnuts ; and each of them contains three round white 
kernels, about the size and shape of hazel-nuts, having small stones 
in their interior, around which the fatty matter exists. From the 
kernel of the stone, an oil fit for burning in lamps may be pressed. 
Almost all the candles burnt in the southern provinces of China 
are made from this vegetable tallow, there being very few sheep 
in that part of the country ; but it does not burn so well as ani- 
mal fat. A tree abounds on the Malabar coast of India, termed 
the Piney, which bears a pulpy fruit that yields a large quantity 
of very solid tallow, almost approching wax in firmness, and 
very superior for the manufacture of candles to animal fat. It is 
not applied to that use by the natives, however, who (on account 
of the heat of the climate which prevents the employment of com- 
mon tallow candles,) are accustomed to burn lamps only, which 
are fed with vegetable oil. This vegetable tallow might prob- 
ably be imported in great abundance and at a low rate into this 
country. 

383. The last inflamable substance secreted by plants which 
will here be noticed, is Camphor, which is much used in the com- 
position of varnishes, besides its employment in medicine. Al- 
though chiefly obtained from a species of Laurel growing in the 
East Indies (where it attains the size of an oak) it exists in nume- 
rous plants, especially those yielding aromatic oils. Camphor 
differs in some degree in its properties, according to the way in 
which it is obtained. In general, pieces of the roots are put into 
an iron vessel, within the cover of which (fitted closely down) 
are cords of rice straw. When the lower part is heated, the cam- 
phor is raised into a vapour, and condenses again on the straw 
above. In old trees, however, the camphor is sometimes found, 
on splitting the trunk, to exist in a very pure state, in the form of 
small concretions or tears, in the interior. This camphor under- 
21 



238 



CAMPHOR. OPIUM. 



goes little loss by exposure to the air ; whilst that obtained by 
heat very rapidly evaporates. Besides the uses of this substance 
already noticed, it should be mentioned that camphor is valuable 
as a preservative of specimens of Natural History against the 
depredations of insects ; and the most effectual way of applying 
it, is to have the cases made of the wood of the Camphor tree, 
which is of a white colour, easy to work, and durable. 

384. Opium is the next vegetable secretion which we shall 
here notice ; and this rather on account of its importance in 
medicine, than because of the large quantity produced, which is 
mostly employed in a manner injurious rather than beneficial to 
mankind. Opium is contained, in small amount, in the milky 
juices of many plants ; but especially in those of the Papaveracess, 
or Poppy tribe. The species which yields it most abundantly is 
the White Poppy (P. somniferum ;) but this does not produce 
it in any large amount in temperate climates, and is cultivated in 
Europe chiefly for the oil yielded by its seeds (§ 371.) The juice 
is obtained by making incisions in the capsules or seed-vessels 
(commonly termed heads) whilst they are quite green ; and that 
which hardens upon them is scraped off. Many kinds of opium 
are known to the importers of drugs ; but their difference only 
results from the varieties of climate in which they are grown, and 
from the mode in which the juice is obtained and prepared. 
Some kinds are very much adulterated. More opium is now 
raised in Hindostan than in any other country ; and the principal 
demand for it has been in China. Opium is a substance of very 
compound nature. A large proportion of it consists of a gum 
soluble in water ; there is also, however, a small quantity of 
resin and of caoutchouc. The ingredients which act so power- 
fully on the animal body, however, constitute but a very small 
proportion of the whole. The most important of these are two 
substances of an alkaline character (being capable of uniting with 
acids to form a salt) which are named Morphia and Narcotine. 
The properties of the first, of these are directly sedative; that is, 
it causes sleep or the relief of pain, without any previous excite- 
ment. The first effect of Narcotine, on the other hand, is to 



MISUSE OF OPIUM. COLOURING PRINCIPLES. 239 

stimulate. These alkalies exist in the opium in combination with 
a peculiar vegetable acid termed the meconic; and they are se- 
parated by chemical processes, since, in order that their medicinal 
effects may be most advantageously produced, it is desirable to 
administer them separately. 

385. The chief consumption of opium, however, is unfortu- 
nately not for the purpose of curing disease, or of relieving pain, 
but for the production of a species of intoxication, the constant 
indulgence in which has a great tendency to degrade the mind 
and to enfeeble the body. The quantity necessary to produce 
the desired effect increases with habit ; so that the confirmed 
opium-eater often takes as his single dose, repeated many times 
through the day, a quantity sufficient to poison any one unaccus- 
tomed to its use. The practice of taking opium often commences 
with the occasional use of it for the purpose of allaying or procu- 
ring sleep ; and those who are obliged to have occasional recourse 
to it for this purpose, should be on their guard against taking it 
more frequently than is absolutely necessary. For such per- 
sons, morphia is the most desirable medicine ; since it produces 
more completely the effects they desire, without that excitement 
to the nervous system which leads to the employment of it as a 
source of pleasure. The quantity of this drug annually consumed 
in England may be stated at about 35,000 lbs. ; whilst that which 
has been introduced into China, in spite of the laws which pro- 
hibit it, has for some years averaged more than 3^ millions of 
pounds, the value of which considerably exceeded that of the 
tea exported. The quantity seized by the Chinese government 
in March, 1839,, was upwards of three million pounds. 

38G. We shall next notice some of the principal colouring 
matters secreted by plants. On these are dependent the varied 
hues so beautifully and abundantly distributed through the vege- 
table kingdom ; of which some at once delight the eye of man, 
whilst gazing upon the garden, the meadow, or the forest ; whilst 
others, extracted from the interior, even of plants of the dullest as- 
pect, contribute to his comfort and luxury in various ways. The 
colouring secretion most universally diffused through plants, is that 



240 



COLOURS OF PLANTS. — CHROMULE. 



termed chromule, on which the colour of all green parts depends. 
It is found in little grains, which adhere to the inside of the cells 
beneath the cuticle ; and the formation of it is due, as formerly 
stated (§ 288,) to the influence of light in fixing carbon from the 
atmosphere. The brightness of this green colour soon disappears 
after the death of the part ; and it is not unfrequently seen to alter 
its hue, whilst vital actions are going on in it. Thus the leaves of 
many trees, as the Lombardy Poplar, change to yellow in autumn, 
long before their fall ; whilst others, as the Beyberry, Sumach, &c., 
turn to red. This alteration is due to an increased absorption 
of oxygen, which is no longer given out by day ; and the chromule 
may be artificially converted, by the action of acids, first into a 
yellow and then into a red. The red colour of many flowers pos- 
sesses the same properties as that of leaves when thus altered ; and 
this fact will possess a higher degree of interest, when it is shown, 
as it hereafter will be (Chap. XII.,) that the leafy parts of flowers 
have the same general structure as leaves, and often differ very 
little from them. It is farther probable that all the colours of 
flowers are caused by the presence of chromule, altered by various 
chemical means ; in all instances it may be seen that these colours 
exist in the same parts, — namely, minute globules contained 
within the cells. It has been observed that the colours of many 
flowers may be greatly changed by cultivation ; in some species, 
as the Dahlia and Poppy, great varieties occur without any ob- 
vious cause, — the seeds of the same parent, raised in the same 
soil, producing flowers of extremely different hues; whilst in 
other cases, the hue is manifestly influenced to a great degree by 
the nature of the soil. It has been ascertained, however, that 
these varieties are not beyond the control of general principles. 
The colours of flowers may be arranged in two series, as under. 
Green. 



Blue series. 



i 

i 



Greenish-blue 
Blue 

Violet- blue 
Violet 
Violet-red 



Yellow-green 
Yellow 

Orange- Yellow 
Orange 
Orange-red 



> Yellow series. 



Red. 



COLOURING PRINCIPLES. BLUE DYES. 241 

Flowers belonging to the yellow series may pass into red or white 
with all intermediate shades, but never into complete blue ; — this 
is the case with the Dahlia, the original colour of which was yel- 
low, and with the garden Ranunculus, which presents every 
gradation from red to green. On the other hand, the Hyacinth 
passes through all the gradations in the blue series, from green, 
through blue, to red ; but never becomes yellow. 

387. The colouring matter of rapidly-growing parts has sel- 
dom sufficient permanence, when removed from the plant to ren- 
der it valuable for the purpose of staining cloth, &c; and the 
substances used for this purpose in the arts of the dyer, calico- 
printer, &c. ; are chiefly obtained from the heart- wood, roots, or 
bark ; sometimes, however, from the softer parts, as the leaves 
and fruit. The principal dyes, by the combination of which all 
varieties of shade may be produced, are blue, violet, red, yellow, 
fawn, and black, and substances yielding all these are produced 
in abundance by different tribes of plants. 

388. Of all the blue dyes, Indigo is the most important. This 
is obtained from the juices of several different species of plants, of 
which some grow in almost all parts of the torrid zone. These 
plants are raised from seed, and are of very rapid growth,— being 
ready for cutting at the end of two months. A subsequent growth 
from the same roots is again ready for the sickle in six or eight 
weeks ; and more may be subsequently obtained. In India, it is 
not considered advantageous to obtain more than four crops from 
the same seed, as the produce of each is less than that of the pre- 
ceding ; but in Egypt, the seed is sown only once in seven years, 
and two crops are obtained in each year. The indigo, which seems 
to be nothing else than the chromule of the plant, is usually ex- 
tracted by fermentation. The plants are laid in a vat, and covered 
with water; and in about 18 hours they begin to swell, and to give 
off a large quantity of gas, — the water at the same time acquiring 
a green tinge. This process is allowed to go on, until the colouring- 
matter of the vegetable tissue has been entirely yielded to the 
water; but if it continue too long, so that any putrefaction take 
place, the dye is destroyed. The fluid is then drawn off into 

21* 



242 INDIGO. 

another vat, where it is violently agitated, for the purpose of sepa- 
rating the pulp from the water. The former consists of little grains, 
which, during the process, turn from green to blue by attracting 
oxygen from the air ; and, by farther processes, it is dried into a 
solid mass, constituting the indigo of commerce. Nearly all the 
indigo imported into Britain is produced in the East Indies ; its 
amount averages about seven millions of pounds every year, of 
which, however, more than half is exported again, chiefly to the 
North of Europe and Italy. Owing to the great variation in the 
productiveness of the crops, the price of Indigo is almost constantly 
changing. In the season 1 824-5, it was nearly lis. 6(/. a pound ; 
whilst in the season 1829-30, owing to an over-abundant supply, 
it was only 4s. id. At the former rate, the value of the average 
quantity annually imported would be about four millions sterling: 
and at the latter, scarcely above one and a half. 

389. This valuable dye has so strong an attraction for almost 
every kind of fibrous texture, whether animal (as woollen or silk,) 
or vegetable (as linen or cotton,) that it will impart to it a per- 
manent colour without the assistance of a mordant * In order to 
apply it, however, it must be dissolved in water ; and this can 
only be accomplished by a change in its chemical nature, which 
restores it to its original yellow-green colour; the stuffs, after 
being dyed, change again to blue by exposure to the air. This 
process appears to injure, in some degree, the durability of the 
colour ; and it is preferable to apply the dye when first separated 
from the plant. The brilliant blue cloths of Africa and China, 
which are superior to those of any other part of the globe, are 
produced in this manner. 

390. The juices of several plants, growing in the different coun- 
tries of the East, are used by the natives of those countries in the 
same manner as Indigo, and might probably furnish a good substi- 
tute for it, if prepared with sufficient care. The use of Indigo as a 

* Mordants are substances used in dying and calico-printing, to hold to- 
gether the particles of the texture dyed, and those of the dyeing material, 
when these have not a sufficient attraction for each other. If not so 
united, many colours would be washed off as readily as they are laid on. 



INDIGO, WOAD. LOGWOOD. 243 

dye, on a large scale, is comparatively recent. It was not until 
long after the discovery of America that it was commonly 
employed in England ; and the use of it was forbidden by the 
governments of some European countries, from the fear that it 
would supercede the use of Woad, which was then very exten- 
sively cultivated. This dye was known to the Ancient Britons, 
who stained their bodies with it ; and it was the principal blue 
dye, until the introduction of indigo. Its colour is much less 
lively than that of indigo, but it is more durable ; hence it is 
commonly employed in union with that and other dyes, but 
seldom now by itself. Woad (7 satis Saliva) is cultivated in 
many parts of Europe ; and is grown in considerable amount in 
Lancashire. Its stem is about three or four feet high, and about 
half an inch in diameter; it divides into many branches, which 
are loaded with leaves. It is cut down with a scythe when the 
flowers are about to appear; and afterwards at intervals of about 
six weeks; — three or four crops being usually obtained in one 
year. The plants are first washed ; and then dried in the sun, 
without which they will begin to putrefy, their green colour 
turning black. They are then conveyed to a mill, where they are 
ground into a paste. This paste is afterwards subjected to several 
processes for the purpose of drying it. It is finally used nearly 
in the same manner as indigo ; with which, indeed, its colouring 
matter, if extracted in the same manner, is found to be nearly 
identical. 

391. A Violet hue is easily given to cloth, by mixing blue 
and red dyes in any required proportion ; but there are some 
plants which yield a violet or purple dye, without any admixture. 
The chief of these is Logwood, the produce of a tree growing in 
the bays of Campeachy and Honduras, — the native country of 
the Mahogany. When Logwood was first introduced into this 
country as a dye, the use of it was forbidden by Government on 
account of its "deceitful" character; the colour it communicated 
being fair to the eye, but speedily departing. The art of fixing it 
by mordants, however, being afterwards discovered, this sub- 
stance came into general use ; and it is now imported largely from 



244 LOGWOOD. — MADDER, BRAZIL-WOOD. 

Jamaica, as well as from its original country. The part which 
yields this dye is the heart-wood; this is hard and heavy, in 
consequence of the amount of secreted matter contained in it ; 
and it yields its colour readily to water, when this is boiled upon 
its chips. The deep violet or purple hue of the fluid first changes 
to a yellowish tint, and finally becomes black ; but this change may 
be prevented by the use of proper mordants. The chief use of 
this substance, however, is in dyeing black, and producing all 
shades of gray. The quantity imported into Britian in 1839 was 
23,000 tons, the value of which was above £180,000. 

392. The principal Red dye obtained from the vegtable 
kingdom is Madder, the produce of the Rubia tinctoria, a plant 
which grows naturally in the Levant, and is cultivated with 
success in the South of Europe ; its cultivation does not answer 
in England. The colouring-matter is obtained from the roots, 
and is not sufficiently formed until the third year ; the roots are 
taken up in the autumn, after the leaves have fallen off. 
They are then carefully cleaned, dried, and reduced to powder. 
A great variety of colours, varying from lilac to black, and from 
pink to deep red, may be produced by the application of different 
mordants to the stuff, before it is placed in the madder. These 
are partly due to the intermixture in this substance, of two 
distinct colouring principles, a fawn and a red. The latter if 
separated from the other, is much more brilliant ; and various 
processes have been devised for the purpose. The best of these 
requires that undried roots should be employed; and these are 
largely imported into this country for the purpose. The quantity 
of madder employed in Britain in 1838 was upwards of five 
thousand tons; and of the roots, more than four thousand. The 
value of these would be together £600,000. 

393. Another valuable Red dye, is obtained from the wood of 
the Gcesalpinia crista,* commonly known as Brazil wood. 
Though abundant in that part of South America, the tree is a 

* An allied species of this tree, the C. pluviosa, also a native of Brazil, is 
remarkable for a constant flow of water from the points of leaves, which 
falls down in drops, like a shower of rain. 



BRAZIL-WOOD, ARCHIL, ALKANET. 245 

native of other parts of the world; and it was known under its 
present common name before the discovery of that country. And 
in fact, the portion of that continent which bears the name of 
Brazil, was so named in consequence of the numbers of these 
trees which were found growing there. As in the case of Log- 
wood, it is only the duramen Q 131.) of this tree which is of any 
service ; the remainder being colourless. • The colour obtained 
from this wood is brilliant ; but is not so permanent as that of 
many other substances. It is generally used to heighten the effect 
of other dyes. Red ink is commonly made by boiling this wood in 
beer, wine, or vinegar, to which alum has been added. Of late years 
the consumption of this wood in Britain has much diminished ; 
whilst that of another kind, termed Peach-Wood, or Nicaragua - 
wood (so named from the Gulf of Nicaragua, whence it was first 
imported into England) has greatly increased, so as to be now 
nearly double the first. The colour obtained from it, is brighter 
and more delicate than that yielded by Brazil- Wood. 

394. Another red dye, now largely employed in England, is 
obtained from a Lichen, commonly termed Orchilla, which 
abounds in the Canary and Cape de Verd Islands, and which is 
sometimes found (though of inferior quality) on the rocks of 
Guernsey and the Isle of Portland. The plant is usually im- 
ported without any preparation ; and it is afterwards dried and re- 
duced to powder, and altered in its character by chemical processes, 
the result of which is a deep red powder known as Archil or cud- 
bear, the infusion of which is of a crimson colour bordering on vio- 
let. It is seldom used by itself, as its colour is not permanent; but 
it is chiefly employed to give a brightness to the hues of stuffs dyed 
with other substances. Several other red dyes might be enume- 
rated, which are used in small quantities for particular purposes. 
Among the most important of these is A/kanet, which is obtained 
from the roots of the Anchusa linctora, a native of the Levant 
and the warmer parts of Europe, but grown also in England. 
This colouring principle is not soluble in water; but it gives a 
deep red colour to oils, wax, and unctuous substances. It is con- 
sequently used chiefly to colour oils, ointments, lip-salves, &c ; 



246 YELLOW DYES | WELD, QUERCITRON. 

and it is sometimes applied to the staining of wood, when dis- 
solved in oil. Notwithstanding the apparent insignificance of 
these purposes, above 50,000 lbs. of it are annually imported 
into this country for home use, besides what is raised in Britain. 

395. Many good Yellow dyes may be obtained from plants ; 
and the most important of all those used in Britain is procured 
from a plant of native growth, — Weld or Wold, or (as it is some- 
times called) dyer's weed. This grows spontaneously in many 
parts of the country on uncultivated wastes, and is a very hardy 
plant, preserving its verdure through frost and drought. It is 
nearly allied to the mignionette ; but is a much taller plant, 
attaining the height of three feet before blooming. It takes two 
years to come to maturity, and is gathered whilst the seed is 
ripening. The plants are dried, and then transferred to the dyer, 
who at once extracts the colour by boiling ; there is reason to 
believe, however, that the seeds contain the really important part, 
and that, if they be saved, the trouble which arises from the bulk 
of the whole plant, may be avoided. The colour is also separated, 
in the form of a yellow powder, for the use of the paper-stainers 
who employ much of it. A much larger quantity of weld is used 
in England than is supplied by cultivation, and it is consequently 
imported from abroad. This is much to be regretted ; as there is 
good reason to believe that it will thrive and yield a handsome 
profit on lands so poor as not to be profitably cultivated in any 
other way. 

396. Another very excellent yellow dye is obtained from the 
bark of the Quercus tinctoria, or Quercitron, a species of Oak 
common in America, the timber of which is employed largely in 
building. This bark is employed in the United States for tanning : 
and its colour being considered a defect, this is removed by a che- 
mical process. More than a thousand tons of it, however, are 
annually imported into Britain : and it is here much valued on 
account of the number of different shades of colour which it may 
be made to produce, as well as on account of its superior dura- 
bility. A much greater demand exists, however, for the dye 
termed Fustic, which is extracted from the wood of a species of 



FUSTIC, ARNATTO, SAFFRON, TURMERIC. 247 

Mulberry tree that grows spontaneously in Brazil and the West 
Indies. It does not yield above one-fourth the anlount of colour- 
ing matter obtained from quercitron, and its colour is not so 
lively ; but it is more efficient in combination with some other 
dyes, and is used with indigo to dye Saxon green, and with salts 
of iron for drab. 

397. Jirnatto is another dye of a reddish yellow, employed 
for particular purposes ; it is obtained from the crimson pulp 
lying between the husk and the seeds of the Arnatto tree, which 
is a native of both the East and West Indies. It is brought to 
this country in cakes, which are made by boiling down the pulp ; 
and these are of a brownish red, giving a bright orange, when 
dissolved in water with the addition of an alkali. Its hue 
is not permanent, however ; and it is seldom employed by itself, 
except for giving colour to cheese, for which it is valued on 
account of the ready communication of its colour without im- 
parting any unpleasant flavour or unwholesome quality. One of 
the most beautiful yellow colouring substances is that known as 
Saffron; but it is too expensive to be much employed by dyers. 
Its chief use is in medicinal and culinary preparations, to which 
it imparts its brilliant hue and agreeable flavour. Saffron is the 
product of a kind of Crocus, which is cultivated in England, as 
well as in France and Spain. This plant flowers in October, and 
the flowers are gathered, even before they are full-blown. The 
stigmata, or points of the pistils (§ 434.,) of these flowers, are 
then picked off; and the rest of the flower is thrown by as useless. 
These little bodies, constituting the Saffron, are next very carefully 
dried, and pressed between paper. Its high price results from the 
very small amount of it produced, even on good land ; even when 
the roots are planted thickly, the average quantity for the whole 
three years (beyond which they should not be allowed to remain 
in the ground,) being not above 26 lbs. per acre. Turmeric is 
sometimes used as a substitute for Saffron, the colour it produces 
being very bright, though deficient in durability. This dye is 
procured from the roots of an East Indian plant named Curcuma 
longa, which has also been cultivated in the West Indian Islands 



248 TURMERIC ; — SUMACH. 

with success. These roots are not unlike ginger, either in figure 
or size ; and the dye brought to this country consists simply of 
the roots, either whole or reduced to powder. It is sometimes 
used to give brilliancy to other hues, and is employed as an in- 
gredient in yellow varnishes. Several other plants affording yel- 
low dyes might be enumerated; but the foregoing are the chief. 
It may be mentioned, however, that the clothiers of some parts 
of Lancashire and Yorkshire make use of common Heath for 
their yellow and orange dyes ; this, with a proper mordant, is said 
to produce on woollen cloth a more beautiful colour than either 
weld or quercitron ; but it is not so permanent. 

398. Almost all vegetables contain more or less colouring mat- 
ter, capable of afford ingfawn or brownish hues, inclining to yel- 
low, red or green. The dye chiefly employed for this purpose, how- 
ever, is obtained from the Sumach, a native of the south of Europe 
and of Syria. The shoots of this plant are cut down every year 
close to the root ; and after being dried, they are reduced to pow- 
der by means of a mill. An infusion of this powder yields a green- 
ish fawn colour, which may be altered by mordants. The prin- 
cipal use of Sumach, however, is in dyeing black, in the manner 
presently to be described. The colouring matter of the husks of 
walnuts forms an excellent dye for wool ; and it is much esteemed 
among the French dyers, for the agreeable and durable hues it 
affords without the assistance of mordants. In order to obtain 
this colouring matter, the husks are kept in water for a year or 
two ; after which they give out much more of it than when fresh. 
The Henna-juice, which is employed by the ladies of the East for 
the purpose of staining their nails, is a very permanent brown dye ; 
the colour not disappearing until the substance of the nails is 
changed by growth. It is also employed for dyeing ordinary 
stuffs ; but it has not been introduced into this country. 

399. The vegetable kingdom affords several substances which 
are capable of themselves producing a permanent black dye; but 
a much larger amount of such materials is required than could 
thus be obtained ; and the black colour of our cloths and stuffs is 
procured by a chemical process, of which the vegetable kingdom 



BLACK DYES. — OAK GALLS. 249 

furnishes one important ingredient. This process consists in adding 
gallic acid to a solution of iron ; by which an insoluble bluish 
black substance, the gallate of iron, is immediately formed. If a 
cloth therefore, previously steeped in a solution of iron, be im- 
mersed in an infusion of any vegetable matter containing gallic 
acid, a black dye will be communicated to it. Almost all vege- 
table substances having an astringent taste contain gallic acid ; 
but especially the Oak tribe. It is from the abundance of this 
acid in the Gall-nut (which is an excrescence resulting from a 
kind of inflammation, excited by a wound of the soft tissue of the 
leaves or young shoots by the gall-fly,) that it takes its name. 
Gall-nuts are not, however, formed upon the Oak of this country :* 
but upon a smaller species which grows wild in the countries 
bordering on the Mediterranean. They are usually pounded and 
then boiled in water, in which the cloth is steeped ; and this is after- 
wards placed in the solution of iron (commonly termed copperas.) 
The colour thus communicated is not a deep black, but rather a 
dark blue. It is improved by logwood, which is boiled with the 
copperas ; and the stuff should have been previously dyed of a 
deep blue with indigo. A similar process is employed in the manu- 
facture of common black writing-ink, which essentially consists 
of gallate of iron suspended in water by means of a small quantity 
of gum; and logwood is here also added to improve the colour. 
Galls are imported from the East Indies, as well as from Turkey ; 
but of late years they have been in less demand, in consequence 
of the introduction of another source, from which gallic acid 
may be obtained at a much cheaper rate. This is in the cups of 
acorns of the Velani Oak, a species which grows abundantly in 
Greece, and in the maritime parts of Asia Minor. These cups, 
which do not contain gallic acid in the same proportion as gall- 
nuts, are known in commerce by the name of Valonia; and in 
consequence of their cheapness (being only about one-fifth the 
price of galls) the consumption of them is very great. During 
the year 1830, the quantity of gall-nuts employee I in England was 
2,297 cwt; whilst that of Valonia was 86,638 cwfc Many other 

* The Oak Apples, however, are similar formations. 
22 



250 BLACK DYES. VEGETABLE ACIDS. 

astringent substances may be used as black dyes with iron ; and 
a good deal of the Sumach imported into Britain is used for this 
purpose, as are also walnut husks in France ; the shells of chest- 
nuts, too, have been employed, although not profitably. In India, 
the juice of the fruit of the Myrobalans, which is not unlike a 
plum, is used for dyeing black with iron ; and when the pulpy 
portion is freed from the stone, which is useless, it contains more 
gallic acid than an equal weight of galls, and might be made a 
profitable article of commerce. 

400. From gallic acid we may naturally proceed to speak of 
the other acids which are produced by Vegetables. These are all, 
like the foregoing substances, formed by the plant itself, from the 
elementary bodies it receives as food ; and thus they may be re- 
garded as true products of vegetable secretion, and not as merely 
separated by the plant from the surrounding soil. In this last 
light we must regard the earths and alkalies obtained from plants, 
and not as products of their secreting processes. The acid which 
is employed in largest quantity is the Tartaric. This is obtained 
from the crust that is deposited by wine, when kept a long time ; 
the amount of which depends chiefly upon the degree of acidity 
in the wine. The crust which goes by the name of Argol, chiefly 
consists of tartaric acid in combination with potash, forming what 
is commonly known as Cream of Tartar ; and this requires to be 
purified from its colouring matter and other impurities, before it 
can be employed in the arts. The acid is easily obtained in a 
separate form by chemical processes ; and it is employed for 
many purposes which cannot be answered by the cream of tartar. 
Its chief use is in many processes of dyeing and calico-printing. 

40 1 . Another vegetable acid much used in the arts is the Ox- 
alic, which is well known as a violent poison. From the resem- 
blance of its crystals, in size and general form, to those of Epsom 
salts, it has not unfrequently been administered by mistake, with 
the most dreadful consequences. This acid is found united with 
potash in the leaves of the Wood-sorrel and common Sorrel ; 
and the oxalate of potash is prepared from their leaves in large 
quantities, in Switzerland and the neighbouring countries, where 



oxalic acid; CITRIC ACID. 251 

these plants grow abundantly. Its long needle-like crystals may 
be seen lying amongst their tissues, if a thin section of the stem 
or leaf-stalks be placed under the microscope. The salt is 
known as Salt of Sorrel ; but it is sometimes sold under the 
name of Salt of Lemons, to which title it has no right whatever. 
The acid may be separated from it, as in the former instance ; 
and it is employed for many purposes by the dyer and calico- 
printer ; as well as for removing the stains of ink, iron-moulds, 
&c. which it does without injuring the texture of the stuff. 

402. The acid which gives sharpness to the juices of lemons, 
oranges, limes, and a variety of other fruits, and is known under 
the name of Citric acid, is likewise one which has many important 
uses, besides that of imparting a peculiarly refreshing character 
to these juices. It is largely employed by calico-printers, who 
now usually import their own lemon-juice, and concentrate it for 
themselves. At one time, the citric acid, which is not combined 
in the juice with any earth or alkali, was obtained by chemical 
processes in separate crystals; but it is now found that the impu- 
rities of the juice do not interfere with its use in calico-printing ; 
and it is employed for this purpose almost in its original state. 
For other purposes, however, pure citric acid is required ; and 
this is partly made in Sicily, where the Lemons are abundantly 
produced, and from which island, with the neighbouring conti- 
nent of Italy, the greater part of the juice consumed in Britain 
is imported. Pare citric acid is used in the preparation of the 
best morrocco Leather; for improving a beautiful scarlet dye 
produced by a preparation of tin ; and for altering the hue of 
some colours which are exclusively used in the dyeing of silk. 
Besides its use in the arts, Lemon-juice is very largely used in 
the navy for the purpose of preventing the complaint termed 
scurvy which is very apt to be brought on by the continued use of 
salt meat. During long voyages, a regular allowance is made to 
each man, which he is required to use as a medicine. This, how- 
ever, hasbeen now rendered less necessary than formerly, since the 
art of preserving meats in a fresh state has been brought into gene- 



252 CITRIC acid ; PYROLIGNEOUS acid. 

ral use. Citric acid exists in many of our commonest fruits, — such 
as the cranberry, cherry, red whortleberry, and the hip of the wild- 
briar ; whilst in the red gooseberry, the currant, the bilberry, the 
black cherry, the wood strawberry, and the raspberry, it is mixed 
with an equal proportion of malic acid, which exists alone in 
apples, pears, and other fruits. It is interesting to notice the uses 
of the acids in these situations. It has been formerly stated that 
gum or starch, when acted on by a vegetable acid with a mode- 
rate degree of heat, is converted into sugar ; and this is exactly 
what takes place in fruits during ripening, — which process con- 
sists in the conversion of the starch of the hard unripe fruit into 
sugar, without any diminution in the amount of acid, which is 
sometimes indeed really increased, whilst its taste is concealed by 
the sugar. 

403. One more vegetable acid may be mentioned ; though it 
probably does not exist as such in the substance from which it is 
obtained, but is formed by the heat employed to set it free. 
This is pyroligneous acid, formerly called acid spirit of wood, 
which is procured by subjecting wood in closed iron retorts to a 
strong red-heat ; the vapour that is given off partly consists of 
this acid, mixed with tarry matter, which is separated by a se- 
cond distillation. This acid, which in some degree resembles 
very strong vinegar, is used by the dyer and calico-printer ; and 
it is also employed for making pickles and other culinary pre- 
parations, in which an acid of great strength is required. The 
impure acid has been found to possess, in a remarkable degree, 
the power of checking the putrefaction of animal substances, even 
when applied in very small quantity ; this is due, however, not 
to the acid, but to a certain ingredient in the tarry matter which 
is mixed up with it, and which, when separated under the name 
of creosote, is now well known as a valuable medicine, especially 
for the relief of tooth-ache. The discovery of the influence of 
this substance in controlling putrefaction would be of great value 
if it were not that by no subsequent process of cooking can the 
tarry flavour communicated by it to the meat be got rid of. 



404. Having thus passed in review some of the most impor- 



EVIDENCES OP A DESIGNING PROVIDENCE. 253 

tant products afforded by the secreting processes of plants for 
man's use and benefit, and having been obliged to confess our 
almost entire ignorance of the processes they serve in the vegeta- 
ble economy, we might proceed to the next division of our sub- 
ject ; but it would be wrong not to pause here for a moment, to 
contemplate the important inferences which may be drawn from 
the foregoing details, in regard to the Power, Wisdom, and Good- 
ness of the Almighty Designer. His power is scarcely any where 
more remarkably displayed, than in the immense variety of pro- 
ducts which are elaborated out of the three simple elements — 
oxygen, hydrogen and carbon, by processes, which, as far as we 
can understand them, appear to be of the most simple description. 
His Wisdom is strikingly evinced in the diffusion of these pro- 
ducts over the whole globe ; so that there is scarcely a country 
which does not naturally contain those which may be most useful 
to its inhabitants. And his Goodness is peculiarly manifested in the 
adaptation of these products — the formation of which (we can 
scarcely doubt, although we cannot understand,) must have an 
object as regards the plants themselves — to the use of Man, in 
ministering to those various wants, which have sprung out of his 
condition as a rational being, endowed with higher faculties and 
more varied powers of enjoyment than those of the beasts which 
perish, and yet dependent for the most favourable use of these, 
upon the judicious employment of the means with which a bounti- 
ful Providence has abundantly supplied him. The nourishment 
of man's body in health, his restoration in disease, the clothing 
that covers him, the varied hues which he can communicate to this, 
the colours which delight his eye in the verdant lanscape, or in 
the skilfully painted picture, the odours which refresh his senses, 
the timber of which his habitations, his manufactories, his ships, 
are partly or wholly constructed, — these are but a few of the 
provisions which the benevolence of the Creator has made lor his 
comfort, in the organization of the Vegetable World. Who, then, 
shall say that it is less fertile in the evidences o( a besigning 
Providence, than the Animal Creation ! 



CHAPTER XL 

OF THE PRODUCTION OF LIGHT, HEAT AND ELECTRICITY BY 
PLANTS. MOTIONS OF PLANTS. 

405. It has been already stated that, by the operation of these 
agents upon the Vegetable system, are chiefly maintained those 
changes which make up the life of each being. (§ 9.) If Light 
be withdrawn, several of the most important of these are speedily 
checked. If Heat be suspended, all of them directly cease. With 
regard to the influence of Electricity, less is known, and nothing 
can be positively stated. But Light, Heat, and Electricity are 
sometimes produced by plants, as well as required by them as 
conditions of their growth. 

406. There are few instances in which Light is evolved 
from living Plants; but these few are very curious. Many flowers, 
especially those of an orange colour, such as the Sun-flower, Ma- 
rigold, Nasturtium, &c. have been seen to disengage light in se- 
rene and warm summer evenings, sometimes in the form of sparks, 
sometimes with a steadier but more feeble glow. Light is also 
emitted by certain species of Fungi, especially those which grow 
in moist and warm places, where light is entirely excluded, as in 
the depths of mines. The light is perceived in all parts of the plant 5 
but chiefly in the young white shoots. It ceases if the plant be 
deprived of oxygen, either by being placed in a vessel from which 
the air has been exhausted, or in some other gas ; and it re-ap- 
pears when the plant is restored to air. No luminousness is per- 
ceived after the death of the plant. It would seem probable, there- 
fore, that this extrication of light is in part connected with that con- 
version of oxygen into carbonic acid, which, as already mentioned, 
takes place very rapidly in flowers and in the whole substance of 
the Fungi (§ 290,) and which may be regarded as a sort of slow 
combustion. An evolution of light has also been observed to take 
place from dead and decaying wood of various kinds, particularly 
that of roots ; and also from Fungi whilst decomposing. This cor- 
responds with the luminousness of certain animal bodies after death. 



HEAT OF ANIMALS AND PLANTS. 255 

407. It is well known that the higher Animals alone possess 
the power of keeping the temperature of their bodies up to a cer- 
tain fixed standard ; and. that in the lower tribes the heat of the 
body varies with that of the atmosphere, being frequently but a 
very little above it ; so that these, giving to the touch a sensation 
of cold, are termed cold-blooded animals. Still, they have some 
power of generating or producing heat, which is shown by their 
power of resisting the influence of extreme cold for a long time. 
In regard to Plants, much doubt has been entertained at different 
times, whether they could be said to have a proper heat or not : 
or whether their temperature is not entirely dependent upon that 
of the atmosphere. But this doubt has resulted from a very li- 
mited view of the processes of the Vegetable Economy, against 
which it is desirable to guard the young reader. 

408. It will be stated, under the head of Animal Physiology, 
that the production of heat is entirely dependent upon the conver- 
sion of Oxygen into carbonic acid, by its union with the carbon 
thrown off in respiration ; and just as the rapid combustion of 
charcoal in oxygen gives out a great degree of heat, so does the 
slower process of union, in which the respiration of human beings 
really consists, disengage heat more gently. Now in Plants this 
process of respiration takes place so slowly (in comparison with 
animals,) and from a surface so openly exposed to the atmosphere, 
that it could scarcely be expected that there should be any sensi- 
ble elevation of the temperature of the part from this source ; es- 
pecially when it is considered that a constant loss of heat is 
taking place by evaporation* 

409. Some recent experiments, however, made with an appa- 
ratus that would indicate extremely slight changes of temperature, 
have proved that the process of respiration in plants is accompanied 
by a disengagement of heat; but in order to establish this, it was 

* This fact is readily understood by pouring a little water, a little spirit 
of wine, and a little ether, one after another, upon the haek of the hand. 
Although they may have been all of the same temperature, the hand i> 
cooled least by the water, more by the spirit, and most by the ether. — in 
proportion, in fact, to the rapidity with which these fluids respectively 
pass off in vapour. 



256 PROPER. HEAT OF PLANTS, 

necessary to compare the temperature of a living plant with that 
of a dead one having the same proportion of moisture at its sur- 
face ; since in this way only could the true effect of respiration in 
producing heat be known, whilst the evaporation was continu- 
ally preventing the manifestation of it, by cooling the surface. In 
this manner it was found that the heat of the surfaces of plants 
is raised by respiration from 1 to 2j degrees above what it would 
otherwise be. 

410. It has long been observed that the interior of large 
trunks possesses a temperature more uniform than that of the 
surrounding air ; being cooler than the atmosphere in summer, 
and warmer in the winter. There are at least two causes of this 
occurrence. Wood is a slow conductor of heat ; thus, if a piece 
of stick and a rod of iron of equal sizes have one end heated in 
the fire, the farther end of the stick will be nearly cold, whilst 
that of the iron is too hot to be handled. Farther, the conducting 
power of wood is still less across the grain (or through the stem) 
than with the grain (or along the stem ;) so that changes in the 
external air will not readily affect the centre of a large trunk ; 
and, accordingly, it is found that, the larger the trunk on which 
the observation is made, the greater is the difference between its 
state and that of the air. The other reason is, that some motion 
of the sap takes place even in winter ; and the fluid taken up by 
the roots principally comes from a depth in the ground, at which, 
from the bad-conducting power of the soil, the temperature is 
nearly uniform throughout the year. 

41 1. The evolution of heat by Plants is most evident at those 
periods of their existence, in which an extraordinary quantity of 
carbonic acid is formed and given off This is the case during the 
germination or shooting-forth of seeds ; and though the heat pro- 
duced by a single seed is too soon carried off by surrounding 
bodies to be perceptible, it accumulates to a high degree when a 
number are brought together, as in the process of malting (\ 283,4.,) 
in which the thermometer has been seen to rise to 110°. The 
same may be said of that other period of vegetation, in which an 
extraordinary amount of carbonic acid is evolved, — that of flower- 
ing (§ 285.) It is evident that, from the little substance of the 



HEAT OF SEEDS AND FLOWERS. 257 

parts thus heated, and the large amount of surface they expose to 
the air, the heat will be carried off by the atmosphere almost as 
rapidly as it is produced. Still in some flowers, a considerable 
amount of heat can be proved by the thermometer to be disen- 
gaged ; thus, a Geranium has been found to possess a heat of 
87°, when the air around was at 81°. 

412. As in the case of seeds, however, the production of heat 
is most sensible, when a number of flowers are crowded together; 
and this is still more the case when they are enclosed in any 
general covering, as are those of the Arum family. In these, the 
flowers are small, and are very closely set upon a stalk, which is 
called a spadix; and the whole cluster is surrounded by a large 
leafy sheath called a spathe. It is in these flowers that the size 
of the fleshy disk is the most considerable, and the quantity of 
carbon to be united with oxygen is therefore the greatest ; and 
the combination of this cause with the other, occasions the tem- 
perature of the clusters to be raised very high. A thermome- 
ter placed in the centre of five spadixes has been seen to rise 
to 111°, and one in the centre of twelve to 121°, — while the 
temperature of the external air was only 66.° The increase 
of temperature commences with the opening of the flower; 
and it is greatest at the time of the shedding of the pollen 
(Chap. XII.) 

413. That the development of heat in these cases is owing to 
the conversion of carbon into carbonic acid, is proved by two 
kinds of experiments. In one, the cluster of flowers was placed 
in pure oxygen, by which this change was performed much more 
rapidly than in common air ; and the heat given out was much 
greater than that evolved by a flower-stem at the same stage in 
common air. On the other hand, a spadix being put into 
nitrogen (the gas which forms the greatest part of common air, 
and seems to have for its object to dilute the oxygen, which by 
itself would be too powerful for the support of animal and vege- 
table life,) the formation of carbonic acid was altogether checked, 
and no heat was given off, although the opening of the (lower. 
and the shedding of the pollen took place to all appearance as 
usual. 



258 DEVELOPMENT OF ELECTRICITY BY PLANTS. 

414. So little has been satisfactorily ascertained regarding the 
connexion of Electricity with the processes of Vegetable growth, 
that it seems undesirable here to dwell upon the manifestations of 
this agent which sometimes occur. It may be stated, however, 
that, whilst on the one hand, the condition of the atmosphere in 
regard to Electricity has evidently a striking influence on the 
rapidity of their growth (some plants having been known to 
increase in the most extraordinary manner during thundery 
weather,) the electricity developed by the changes which take 
place in the economy of plants has probably a very powerful 
influence on the condition of the atmosphere. It is well known 
that by all chemical changes, such as occur in every process of 
vegetation, — from the absorption of the crude sap, to its final 
conversion into the substances which are to remain fixed or per- 
manent through a long series of years, — electricity is produced. 
Farther, the mere evaporation of water from the surface of the 
leaves will do the same ; and thus a constant series of changes 
in the electric state of plants will occur, which will communicate 
themselves to the atmosphere. 

415. The general electric state of plants is found to be that 
termed negative; and if any circumstances cause the atmosphere 
to be positively electrified through a considerable space, some 
great commotion of the elements is not unlikely to take place. 
Hence, the dreadful hurricanes, which occasionally devastate the 
West Indian islands, may be in some degree accounted for. The 
evaporation of the water from the surface of the surrounding 
ocean tends to make the air above it positively electrical ; and, 
this, too, at the very time when the brilliant light and genial 
warmth of the sun is causing the vegetation of the land to pos- 
sess an opposite condition. " How wonderful," it has been re- 
marked, " are the operations of nature. The silent and peaceful 
growth of a vegetation whose splendour fascinates the eye, de- 
velopes an agency, which, opposed to that produced by the rapid 
but unobserved evaporation from the surface of the surrounding 
ocean, tends to load the atmosphere with conflicting elements, 
from the depth of whose strife issues thunder proclaiming the 
approach of the hurricane and tornado." 



259 



OF THE MOTIONS OF PLANTS. 

416. The gradual movements of the parts of plants, which 
occur as a part of the natural changes involved in their growth, — 
such as the extension of their roots beneath the ground, and the 
elevation of their leaves and flowers by the upward extension of 
their stems and branches, — have been already noticed, and the 
causes which influence them have been assigned, as far as our 
knowledge of them extends (§ 107, 309.) A curious experiment 
has been recently performed, which proves in a remarkable man- 
ner the influence of light on the direction of the growth of these 
parts. Some seeds of Cabbages, Mustard, and Kidneybeans, 
were placed in Moss ; and were so arranged, that the only light 
they could receive was from a mirror, which threw the solar rays 
upon them from below upwards; the natural direction of their 
growth was in this manner completely changed, — the stem being 
sent downwards, and the roots upwards. 

417. We have here to notice, however, another set of move- 
ments displayed by Plants, in which an evident change of place 
occurs whilst they are being observed for a short time. One of 
these is known as the sleep of plants, from the circumstance of its 
generally occurring in the evening. This consists sometimes in the 
folding-together of the leaves, in other cases in their drooping, and 
occasionally in their clasping the stem ; it is most displayed in 
Leguminous plants having pinnate leaves (§ 238 ;) and in them 
the lateral leaflets commonly fold together, whilst the leaf-stalks 
are bent downwards on the stem. Many flowers, also, exhibit a 
regular movement, of the same description; — closing together at 
night, and unfolding in the morning. There are a few species, 
however, which unfold at night and close during the day. There 
are some, too, which close during the day, when the sky is over- 
cast and a storm is threatened. These changes seem almost en- 
tirely dependent upon the degree of light to which the plant is 
exposed; for they may be made to take place at the contrary 
periods, by keeping the plants in a darkened room during the 
day, and placing them at night in strong lamplight It is usually 
some little time, however, before they become accustomed to the 



260 



MOTIONS CAUSED BY LIGHT AND MOISTURE. 



change ; and their movements are at first irregular. The mode 
in which light produces these movements has not yet been as- 
certained ; but it can scarcely be doubted that it is by its influ- 
ence on the exhalation of fluid from the soft tissues, on one side 
of the bending part more than on the other. Supposing that the 
part were otherwise bent, the influence of light upon the cells of 
the convex side would cause them to contract, and thus straighten 
it, — a change which we shall presently see to be elsewhere ef- 
fected by another cause acting in like manner. Whilst, if the 
part were straight in the dark, so that the leaves were erect, and 
the flowers expanded, the influence of light acting more on one 
side than on the other would cause it to bend. 

418. The influence of water, or of varying degrees of moisture 
in the atmosphere, seems often to produce movements in the living 
plant, as well as in dead portions of its tissues. It is in this way 
that the closure and unclosure of the Rose of Jericho and the 
Lycopodium of Peru are occasioned, — the one by drought, the 
other by the contact of fluid. This is easily accounted for by sup- 
posing that the cells on one side are larger and have thinner walls 
than those on the other ; and these will, therefore, be most easily 
distended when placed in water, and will soonest lose their fluid 
in drying. The beards of the Geranium and Wild Oats curl up in 
dry weather and straighten in damp ; those of most other plants 
perform the contrary movement. Such parts of plants are often 
used in the construction of hygrometers, to indicate the amount 
of dryness in the atmosphere, to detect dampness in beds, &c. 

419. Some of the most interesting among the vegetable move- 
ments are those concerned in the deposition of the seed. The Bal- 
sam termed Impatiens noli-me- Lang ere has a seed-pod or capsule 
formed by two halves or valves, which when the seed is ripe, 
suddenly separate from one another and curl inwards scatter- 
ing the seed to some distance. Now an examination of the tis- 
sue of these valves shows that their outer part consists of much 
larger cells than the inner, and that the fluid contained in it is 
the densest. By the laws of Endosmose (§ 118,) therefore, the 
fluids contained in the tissue of the interior will have a tendency 
to pass towards the outside, and will distend its vesicles still 



MOTIONS OP SEED-VESSELS. 261 

more. This distention of the outside layer will manifestly give the 
valves a tendency to curl inwards ; just as when two thin plates 
of metal, which expand unequally by heat, are soldered together, 
and, heat being applied, the compound plate bends towards the 
side which expands least. This tendency continues to increase 
up to the time when the seed is ripe ; and it is then so power- 
ful as to cause the separation of the valves from each other, and 
to occasion the rolling inwards of each. Now it has been found 
that, if the valves be placed in a fluid more dense than that 
which the valves contain, such as sirup or gum- water, the fluid 
will be drawn off from their cells according to the same law of 
Endosmose ; and the cells on the exterior will be emptied soon- 
est, on account of their being larger and fuller than the other ; so 
that the valves become straight, and even curl outwards. But if 
they be put into water, the Endosmose, still taking place towards 
the side on which the fluid is densest, — namely the interior of the 
cells, — will distend them still more, and will cause the valves to 
curl inwards more powerfully than at first. Another instance 
of movement with the same object, which may be explained in 
a similar manner, is that of the seed-vessel of the common 
Squirting-Cucumber {Momordica Elaterium.) This when ripe, 
very readily separates from its stalk ; and its pulpy contents 
are violently forced out from the aperture thus left. The pulpy 
matter surrounding the seeds occupies the centre of the fruit, 
and by its own increase in amount, distends the cavity; the elas- 
ticity of the walls, therefore, occasions their violent contraction, 
when an aperture is formed in any way, by which the distention 
is relieved. 

420. Such explanations, however, will by no means account 
for all the evident movements of plants ; and it is necessary to 
suppose their living tissues to be endowed with a property termed 
contractility, by which they are enabled to contract upon the ap- 
plication of a stimulus, just as do the muscular fibres of animals. 
The vegetable kingdom affords many examples of this kind of 
contraction. Thus, if the leaves of the common Wild-Lettuce be 
touched, when the plant is in flower, the part will be covered 
23 



262 CONTRACTILITY OF VECETABLE TISSUES. 

with milky juice, which is forced out through the stomata by the 
contraction of the cells or vessels beneath. Again, in the flower 
of the Berberry, if the base of the stamen be touched with the 
point of a pin, the filament or stalk will bend over, so as to strike 
its top against the style or central pillar of the flower. This 
movement will hereafter be seen to be connected with the 
process of fertilization; and it must be frequently caused by the 
contact of insects, which thus assist in that function. There is a 
curious New Holland plant, named Stylidium, sometimes culti- 
vated in green-houses in this country, which has a tall column 
rising from the centre of its flower, and consisting of the stamens 
and style united ; this usually hangs down over one side of the 
flower, but if it be touched ever so lightly, it starts up with a jerk, 
and rapidly swings over to the opposite side. 

421. One of the most interesting of all the vegetable move- 
ments, however, is that displayed by the Sensitive plant (Mimosa 
pudica.) This is a Leguminous plant of the Acacia kind, which 
has its leaves very much subdivided into leaflets. When spread 
out in sunshine, they present no peculiarity of appearance ; but 
at night they fold together as in sleep, more completely perhaps 
than the leaves of any other plants. If, when expanded, one of 
the leaflets be slightly touched, it will close towards its fellow ; the 
neighbouring leaflets will presently do the same ; the vein upon 
which these are set will bend downwards, and meet the one on 
the opposite side of the midrib ; the midrib itself will afterwards 
bend down upon the stem ; and, if the plant be in a very irritable 
condition (from its functions being in a state of great activity) the 
other leaves are sometimes affected in a similar manner. The ex- 
planation of this very curious phenomenon requires that the struc- 
ture of the parts concerned in it should be explained. It is evi- 
dent that the cause of the movement must be in some way pro- 
pagated from the part touched, to the parts where the change 
actually takes place, — namely the points where the leaflets join 
the veins, the veins come from the midrib, and the midrib, from 
the stem. At every one of these points, there is a little swelling 
or intumescence, formed of very spongy cellular tissue, and con- 
taining a great deal of fluid in its cells. If the under side of the 



MOVEMENTS OF SENSITIVE-PLANT. 263 

intumescence at the foot of the leaf-stalk be touched, its vesticles 
being very irritable, contract and force out the fluid they contain ; 
and this necessarily pulls down or depresses the leaf-stalk and 
all that it carries. If, on the other hand, any thing distend the 
cells on the upper side of the intumescence, the leaf-stalk is pushed 
down, as it were, in a similar manner. The intumescence at the 
origin of each vein, and at the base of each leaflet, seems to pos- 
sess the same properties in a degree proportional to its size, and 
they are all connected together by the vessels and woody tubes 
of the midrib and veins. Now, when the tissue of any of the 
leaflets be touched, it appears to contract in the same manner as 
does that of the Wild-Lettuce ; but instead of squeezing out its 
fluid upon the surface, it forces it through the vessels into the 
upper side of the intumescences at the base of itself and its fellow, 
and these leaflets are thus caused to fold down and meet each 
other. The fluid forced out from the under side of their intu- 
mescences is probably carried to the upper side of those at a 
little distance ; and thus the neighbouring leaflets also are de- 
pressed. The depression of the veins upon the midrib, and of the 
midrib or footstalk itself upon the stem, will follow in like man- 
ner ; the extent to which the movement is propagated, being de- 
pendent on the amount of fluid expelled from the lower side of the 
intumescence, in the parts where it has already taken place. 

422. Various other stimulants, besides the touch of a hard 
body, will produce similar effects. Thus, if electric sparks be 
communicated to the lower side of the intumescence, or the rays 
of the sun be concentrated on it with a burning-glass, a simiku 
contraction of its vesicles, and depression of the leaf, will follow. 
In this, as in the foregoing instance, the leaves return after a 
time to their usual condition. Several species of the Acacia 
tribe, growing in warm climates, exhibit corresponding change- 
in a less degree. The closure of the fly-trap of the Dionaea 
(§ 246.) may be probably explained on similar principles ; the 
part here irritated is the tissue at the base of the three thorns on 
each side of the leaf, one of which must be touched in order to 
excite the movement. 



CHAPTER XII. 



OF THE REPRODUCTION OF PLANTS. 



423. The limits which have been set by the Creator to the 
duration of the life of each being that exists at any one time on 
the surface of the globe, would cause the earth to be speedily 
unpeopled, were not a compensation provided in the faculty of 
Reproduction, — or of the formation of a new being similar to 
itself,— possessed by every kind of Plant and Animal. This 
power of creating (as it were) a living structure, with all its 
wondrous mechanism, — possessed, too, in Animals of the facul- 
ties of sensation and thought, and in Man the residence of an 
immortal spirit, — seems at first sight more extraordinary and 
mysterious than any which we elsewhere witness. Yet it is not 
perhaps so in reality. The processes which are constantly taking 
place during the life of each being, and which are necessary to the 
maintenance of its own existence, are no less wonderful, and no 
less removed from any thing which we witness in the world of 
dead matter. When the Tree unfolds its leaves with the return- 
ing warmth of spring, there is as much to interest and astonish, 
in the beautiful structure and important uses of these parts, as 
there is in the expansion of its more gay and variegated blossoms ; 
and when it puts forth new buds, which by their extension pro- 
long its branches over a part of the ground previously unshaded 
by its foliage, the process is in itself as wonderful as the formation 
of the seed that is to propagate its race in some distant spot. Thus 
it is that scientific knowledge heightens our interest in Nature, 
by showing that, in those things which seem most common, 
there are as many sources of interest and instruction, as in that 



SIMPLEST MODES OF REPRODUCTION. 265 

which, from its apparently mysterious character, is usually re- 
garded with more curiosity. 

424. In the lowest plants, the process of reproduction is as sim 
pie as that of their growth. Each single cell of the Red Snow 
(§ 48. Fig. 8.) for example, produces within itself a number of 
little particles ; which, at a certain period, are set free by the 
bursting of the parent-cell which encloses them. These granules 
then gradually enlarge, — deriving their nourishment from the air 
and moisture around ; and in time they acquire the size of the 
parent plant, and in their turn produce a new family within them- 
selves, which at the proper time they set free. A similar process 
takes place in the Yeast-Plant (§ 56. Fig. 10.) In the Confervas 
(5 41. Fig. 6.,) — in which a number of cells are united together, 
end to end, in each filament, — the several cells in like manner set 
free from their interior the little green particles which serve to 
propagate their kind; but the parent cells do not lose their own 
lives in thus sending a new generation into the world ; for, instead 
of bursting, they allow the granules to pass out by a small aperture 
which forms in their walls. The growth of these particles within 
the parent cell may be distinctly traced : at first they are seen 
adhering to its inner wall ; then they separate themselves from 
it, and float in the fluid it contains ; then they are seen to move 
while yet within the cell ; and after they have passed out, they 
continue their motion, even in an increased degree, for some time. 
At last they attach themselves to some fixed object, and their de- 
velopment into new plants then begins. The particle gradually 
enlarges and forms a cell containing fluid; this cell takes an oval 
form, and a partition then appears across it, dividing it into two ; 
one of these is elongated in the same manner, and is again sub- 
divided ; so that at last a complete filament, consisting of many 
cells, is produced ; — this, in its turn, sends out reproductive par- 
ticles from its cells, which go through the same processes. The 
curious movement of these granules (which any one possessing an 
ordinary microscope may observe for himself, by watching the 
reproductive processes in the common Conferva? of our streams) 
has given rise to the notion that they were to be regarded as 

23* 



266 REPRODUCTION IN THE LOWEST PLANTS, 

animalcules at this stage of their existence ; — a notion which is 
only mentioned here to point out its absurdity ; since, whatever 
may be the cause of these movements (which is still obscure,) 
they do not afford any evidence of being guided by Sensation 
and will, of which no real animal can be entirely destitute. 

425. In all these cases, the process of Reproduction is performed 
in a manner as simple as that, which any of the functions of Vege- 
table Life present to us. There is nothing more wonderful in the 
fact that a cell should produce the rudiments or germs of new 
cells, in its interior, than that it should develope additional cells 
which are to form parts of its own structure, (as in the Yeast- 
plant \ 56, and higher plants in general,) from its outside. Each 
may be regarded as a Law of Nature ; — which is only saying that 
it is the mode in which the Creator operates. Now we shall find 
that, in higher plants, the essential part of the reproductive pro- 
cess is really the same — following the same general laws ; and it 
is one of the most interesting results of scientific research, to see 
that things which appear widely different, may often prove to be 
closely connected. We may hence learn a lesson, too, which is 
very useful in the ordinary concerns of life, — not to judge too 
hastily by appearances. Nothing could seem more unlike than 
the production of the seed of some noble tree, from the elegant 
flower, with all its complex apparatus of parts, — and the propa- 
gation of the humble kinds of vegetation we have been consider- 
ing, by the simple conrtrivances just described. And yet it will 
be seen that, although in the former there is much of an addi- 
tional character, subservient to particular purposes, yet that the 
mode in which the germ is at first produced is essentially the 
same. 

426. The first stage of this increasing complexity is seen in the 
higher Sea- weeds; in which, of the large number of cells that the 
whole plant contains, only a small part are appropriated to this 
function. Sometimes these reproductive cells are spread over the 
whole surface of their leaf-like expansion ; but sometimes they 
are restricted to the extremities of the plant. In the common 
Bladder-wrack [Fucus vesiculosus,) which abounds on most of 



REPRODUCTION IN ALGiE, LICHENS, AND FUNGI. 



267 



the shores of Britain, a swelling may be seen at the end of each of 
its divisions, which is distinguished from the rest by its yellow 
colour, when the fructification which it contains is mature. In this 
swelling a number of pores or minute apertures may be distin- 
guished ; and if the substance be cut across, it will be found that, 
beneath each of these pores, there lies a cell larger than the rest, 
and partly separated from it. This cell, when the fructification 
is ripe, passes out through the pore, and soon after bursts, set- 
ting free the minute particles it contains; and these, like the 
granules of the Red Snow or the Confervae, develope themselves 
into new cells, by the multiplication of which a new plant similar 
to the parent is gradually reproduced. Now this cell, thrown off 
from the rest of the structure, and containing reproductive par- 
ticles, which it afterwards sets free, corresponds with what in the 
higher Cryptogamia are called spores. These spores take the place 
of seeds in this division of the Vegetable kingdom. We shall 
hereafter (§43 1> 440, &c,) trace the differences in their structure. 
427. The processes of Reproduction in the Lichens and 
Fungi appear to be as simple as those just described. Cells are 
seen in certain parts of the structure, which differ from those 
composing its own tissue, and which are destined to be cast forth 
from it, when the reproductive particles it contains are mature. 
The immense number of these reproductive cells or spores which 
are contained in the different plants of the Fungus tribe, has been 
already noticed; and the various organs which contain them 
will be hereafter described. One of the highest forms of this 
group is the common Mushroom, in which there is a very dis- 
tinct separation of the fructifying from the nutritive system. The 
spores are contained in a number of little tubes, which are 
arranged side by side in the membrane forming the cap of the 
Mushroom, and in the thin plates (commonly known as the gills) 
which spread from the centre on the under side of this ; whilst 
between this part and the roots is a distinct stem. The whole 
energy of the Fungi seems directed towards the propagation of 
their race; and the duration of life in individuals is usually very 
transient. In Lichens, on the other hand, each individual tie- 



268 REPRODUCTION IN LIVERWORTS AND MOSSES. 

quently exists for many years, and its powers of propagation are 
much inferior. Indeed some Lichens do not form any distinct 
spores ; but multiply themselves by little bud-like bodies, which 
they form in hollows of their surface. In the common Cup-Moss, 
for example, (which is really a Lichen,) these little bodies may 
be seen in the form of a fine greenish powder, in the hollows of the 
cups ; and from these, when they are removed from the parent 
plant, new individuals will spring. 

428. In the Liverworts we find a similar provision, as already 
noticed (§ 32. ;) but here there is a distinct set of organs of fructi- 
fication raised above the general level of the plant, as shown in 
Fig. 4. The little bodies forming as it were the spokes of the 
wheel, are cases containing spores or reproductive cells ; and these 
are scattered, when mature, by a set of elastic spiral filaments 
which lie among them. When it begins to develope itself, the 
spore does not altogether burst and emit the granules it contains, 
as in the Algae ; but its outer coat only ruptures, and a long tube 
projects from its interior, within which new cells are seen to grow, 
taking their origin from the granules or minute germs which the 
spore contained. These cells gradually increase into a leafy ex- 
pansion, from the lower part of which root-fibres proceed ; and 
this in time acquires the appearance of the original plant, and 
forms its own organs of fructification. 

429. The organs of fructification in the Mosses Q 27.) 
are extremely beautiful and delicately-formed; but as the es- 
sential nature only of their reproduction is here to be explained, 
the description of these peculiarities must be deferred. We here 
find a complex provision for the development and dispersion of 
spores, which in themselves resemble those of the Marchantia. 
The little urns, mounted upon long stalks, which are peculiar to 
this group, are furnished with lids, that drop off when the 
spores within them are mature; these spores having been 
developed around a central pillar termed the columella. The 
subsequent changes which take place in the spore nearly cor- 
respond with those described in the last section ; the principal 
difference being, that a number of tubes are put forth instead of a 
single one. Each of these tubes can be perceived to contain some 



REPRODUCTION IN FERNS. 269 

of the little granules which the cell produces within it ; and every- 
one of these is capable of itself forming a perfect plant, as has 
been ascertained by cutting the tubes into several pieces. In ge- 
neral, however, all these go to form one young Moss, — the cells 
which they produce uniting together at an early period ; and thus 
the process is rendered much shorter than if the whole plant had 
to be developed from a single cell. 

430. In the Ferns, again, we meet with another form of the 
same process. The spore-cases are here developed on the backs 
or at the edges of the leaves, and differ in form from those of the 
Mosses ; but the spores which they contain could not be distin- 
guished from theirs. If a Fern-leaf whose fructification has come 
to maturity (as may be known by the brownish tinge of the yel- 
low or orange spots or ridges on its leaves) be placed with its 
underside upon a piece of white paper, this will be found in a day 
or two covered with a very fine brown dust. These are the 
spores which are scattered by the bursting of their cases. The 
process of development of these spores presents several points of 
interest. In its first stages it closely resembles that of the Mar- 
chantia. The outer coat of the spore ruptures, and the inner one 
projects into a long tube ; within which, as well as within the ori- 
ginal cavity, new cells are formed from the germs included within 
it. The first tendency of these newly-formed cells is to grow to- 
gether, and to increase into leaf-like expansion, very much resem- 
bling that of the Marchantia. In the middle of this (which has 
received the name of primary frond,) a knot or protuberance 
gradually makes its appearance; and this is afterwards pro- 
longed above into a sort of stem, and below into a root. From 
this stem, the true leaves or fronds are afterwards developed, 
unrolling themselves after the manner formerly described (§ 25 ;) 
and, when these make their appearance, the primary frond 
decays away, leaving no traces of its existence. In this very 
curious process, we see that the Fern passes, as it were, through 
the stage which is permanent in the Marchantia; but that when 
it attains a higher form, the organ, which was only for a time 
subservient to its existence, decays away. Many instances of 
a similar kind present themselves in the Animal kingdom. Thus 



270 TRANSITORY CONDITIONS OF LIVING BEINGS. 

the Frog comes forth from the egg in a state resembling that of 
a Fish, breathing by gills instead of by lungs, possessing a long 
tail by which it moves itself in the water, and destitute of legs. 
Subsequently legs are produced, which render its tail unneces- 
sary ; and lungs are developed, which perform its respiration more 
effectually than gills ; and these two sets of organs, though they 
permanently exist in Fishes, disappear in the Frogs, as soon as 
they have served their temporary purpose. Corresponding 
changes, hardly less striking than this, take place during the de- 
velopment of every one of the higher animals ; and in every in- 
stance we see that, when a higher form is attained, the parts which 
had their uses in an inferior condition of existence, are cast off as 
cumbrous and unnecessary. How beautifully does this principle 
apply to the history of the development of the human soul! At 
first it is entirely dependent for its activity on the impressions which 
it receives through the bodily frame with which it is connected. 
The calls of hunger, the presence of unaccustomed objects, strong 
impressions upon its senses, first excite its attention ; and all its 
subsequent acquisition of knowledge depends upon similar influ- 
ences. Perfect in their kind as are the organs of sensation by 
which these impressions are communicated, there are still bounds 
to their operation. All that their highest exercise, with the aids 
derived from the most refined ingenuity, can effect in this life, 
serves but to give to the philosophic mind a glimpse of the won- 
ders of Creation ; and there can scarcely be to such a mind a more 
powerful natural* argument in favour of a future state, than that 
which rests upon the vast amount of knowledge of which the 
sources are presented to man, and the insatiable desire for it which 
he possesses, compared with his very limited power of satisfying 
that desire within the short duration of an ordinary life. All ana- 
logy, then, leads to the conclusion, that with the mortal body, the 
soul shall cast away those instruments which are adapted only to the 
present material finite state of existence, and shall be endowed with 

* By this is meant an argument drawn from the Natural World, as 
distinct from the Revealed Word of God. 



REPRODUCTION IN FLOWERING PLANTS. 271 

more direct means of becoming acquainted with those glorious 
truths which here it only sees " as through a glass, darkly." 

431. To return from this digression. — In reviewing the pro- 
cesses of Reproduction in Cryptogamia, we perceive that they 
are every where essentially the same. The spore, or reproductive 
cell contains a number of granules, each of which is capable of 
producing a new cell, at the expense of the fluid which its parent 
contains ; and these new cells are able, either together or sepa- 
rately, to develope themselves into plants similar to their parents, 
without any other influences than those which they receive from 
the light, air, and moisture, which surround them. Now we shall 
find that the real essential difference between the Phanerogamic 
and the Cryptogamic (the flowering and the flowerless) plant, 
consists in this, — that the former possesses a series of organs fitted 
to receive and cherish the germ, and to assist in its early de- 
velopment, of which the latter is destitute ; and that the presence 
or absence of those parts which are ordinarily known as con- 
stituting the flower, is of no primary importance. These parts 
are often absent, without the process of Reproduction being 
thereby affected ; and, on the other hand, there are many flowers, 
which appear perfect to the uninstructed eye, but which are 
totally destitute of fertility. 

432. The parts of a flower essentially concerned in the repro- 
ductive process are the stamens and pistil. The stamens are a 
number of little bodies, having yellow heads mounted on long 
stalks, which are seen around but not in the centre of the flower. 
These stalks are called filaments; whilst the heads are called the 
anthers. Each head is usually seen to be more or less completely 
divided into two parts, which are termed anther-lobes. These 

are commonly united to 
\f gether ; as in Fig. 46. a, b, c. 




d; but sometimes they are 

separated, as at c; and occa- 

Fig. 40. Different forms of sta- sionally only a single lobe is 
mens ; a, lily ; 6, lemna ; c potatoe ; d, pvesent as at f Within the 
berberry ; c, ginger ; /, sage. 



272 STRUCTURE OF POLLEN, ANTHERS, AND OVARIUM. 

stamens are produced a number of minute yellow bodies, usually 
of a globular form, which together constitute the fine dust known 
as the pollen ox farina of the flower. Each grain of pollen, when 
examined with the microscope, is seen to consist of a cell exactly 
analogous to that which constitutes a spore. It has two or more 
coats, which enclose a fluid ; and in this a large number of ex- 
tremely minute granules may be seen with a good microscope. 
These granules are probably the germs of new cells ; being ana- 
logous to those which are sent forth from the Red Snow, the 
Conferva?, and the Yeast Fungi. They may be seen to move 
within the parent cell, or pollen-grain, previously to the time when 
its walls become too thick to allow of their being observed 
through them ; and, when the contents of the pollen-grain are 
mixed with water, they are seen to be constantly performing a 
sort of vibratory motion. 

433. The anthers, or receptacles of pollen, which evidently 
correspond with the capsules or spore-cases of the Cryptogamia, 
burst when their contents are mature, and scatter the grains 
forth. They have various ways of opening ; sometimes they 
split along their length, as at a, Fig. 46 ; sometimes transversely, 
as at b ; sometimes by little openings at their extremity, termed 
pores, as at c ; and sometimes by valves, as at d. These different 
methods are characteristic of different tribes of Flowering- 
plants. 

434. Now the portion of the reproductive system in the 
Phanerogamia, to which nothing analogous exists in the lower 
tribes, is that which is denominated the ovarium or seed-vessel, 
which occupies the centre of the flower, being sometimes 
situated above and sometimes below the point at which the 
leafy parts of the flower arise from the axis which bears 
them. This ovarium is the part in which are formed the ovules 
or young seeds ; and these, after being fertilized in the manner 
presently to be described, ripen into the perfect seeds. Some- 
times it consists of several evident divisions ; in other instances, 
these are united together, more or less closely ; and all mark of a 



STRUCTURE OF OVARIUM, PISTIL, &C. 



273 




Fig. 47. Pistil of Coria- 
ria myrlifolia, showing 



division may even disappear. The ad- 
joining figure represents the centre of a 
flower in which the several parts of the 
ovarium remain separated; three only 
are seen, the others being concealed by 
them. These separate parts are termed 
carpels. Each carpel is surmounted by 
a sort of pillar, termed the style; which 
usually expands at its summit into a 
fleshy surface, called the stigma. When 
the carpels adhere closely together, their 
styles also frequently unite, so as to form 
a single pillar, which sometimes how- 
ever divides again into several branches 



distinct carpels and styles. a t the top. The ovarium with its style 
and stigma, is then called the pistil; and 
sometimes each separate carpel with its own 
style and stigma receives the same appellation. 
An excellent illustration of an ovarium con- 
sisting of many carpels, appearing externally 
single, but each really separate from the 
rest, is the Orange; the rind of this fruit 
is formed by the external part of the flower 
which wraps over the ovarium, whilst the 
juicy part is the ovarium itself, composed of a 
number of carpels adhering together, but not 
so closely united as to prevent their being torn 
apart. The position of the pips or seeds of the 
Orange will give a good idea of the manner 
in which they are usually situated within the 

„ carpels, especially when they are few in nuni- 
iig. 48. Pistil of 
Vaccinium amoe- ber. Sometimes, however, they are attached 
num; a, ovarium; 6, to the whole length f the carpel, f rom ne 
calyx; a, placenta; , , ., ., . ., 

e, ovules; /, pistil; end to the other, as is seen in the common 

and g t stigma. Pea, of which each pod is a separate carpel. 

The portion of the carpel from which ovules arise, is usually 

24 




274 



STRUCTURE OF OVARIUM AND FLOWER. 




thick and fleshy, and is termed the placenta. The section of the 
pistil of the Whortleberry, in the last page, will give an idea of the 
arrangement of the parts in an ovarium whose carpels and styles 
have united. The ovarium «, of this flower is wrapped over, like 
that of the orange, by the leafy portion of the flower itself; which is 
seen to rise beyond it at b. The centre of the ovarium is occupied 

by a thick fleshy placenta, 
.c formed by the union of that of 
the several carpels ; and on this 
the ovules are clustered. Above 
is seen the single style with its 
stigma. Another variety of 
the same kind of structure is 
shown in Fig. 49 ; here the ova- 
Fig. 49. Ovarium of Thamnea uniflora; riumj ft ? is m like marmer en- 
a, calyx; o, ovarium; c, disk; a, ovules. 

veloped by the outer part of 

the flower, a; and the partitions between the carpels have entirely 
disappeared, so that only one central pillar is left, around which 
the ovules are clustered. There is another common form of the 
ovarium of which that of the common 
Heartsease, Fig. 50, may be taken as an 
example : in this the partitions have dis- 
appeared ; but the placentae of the several 
carpels, instead of remaining clustered to- 
Fig. 50. Ovarium of Viola gether, are attached separately to the 
tricolour; a, placenta. walls of the ovarium, as at a. 

435. These two sets of organs are by no means constantly 
united, however, in the same flower. The staminiferous or sta- 
men-bearing flowers are frequently distinct from those which are 
pistittiferous or bear pistils. When they occur on some other part 
of the same plant, it is said to be monoecious (single-housed ;) if on 
a different plant, it is dioecious (or double-housed.) Sometimes 
the same collection of flowers contains some perfect ones, with 
others staminiferous, and others pistilliferous only. There is reason 
to believe that, when either set of organs is not developed the 



at- 




FERTILIZATION OF THE OVULES. 275 

rudiments of it really exist ; for these parts are frequently made 
to appear by cultivation. 

436. If the ovarium be cut into, previously to the opening of 
the flower, it will usually be found to contain a great number of 
the ovules or young seeds. These are at that period quite soft ; 
and their interior is filled up with a kind of pulp, which is en- 
closed in two or more envelopes. These seed-coats do not en- 
tirely cover the central envelope, but leave a small opening, which 
is called the foramen. This opening may be easily detected in 
the perfect seed, although it has there nearly closed up, by soak- 
ing it in water, and then pressing out the fluid that has been ab- 
sorbed, which will be seen to issue from this little orifice. The 
foramen, as will presently appear, has a very important purpose 
in the fertilization of the seed ; which, at the period now described, 
contains no trace of the germ of the new plant. 

437. This germ appears to be conveyed into it from the 
pollen in the following curious manner. The little grains or 
cells, when set free from the anthers, fall upon the stigma of the 
pistil. In general the anthers are situated above the stigma, — 
the stamens being longer than the pistil in flowers that are erect 
or upright, and shorter in those which hang down ; but some- 
times a special provision is necessary for the conveyance of the 
pollen to the stigma, especially in monoecious or d'i'cecious plants. 
This function is often accomplished by insects, which in going 
from flower to flower in search of honey, cover over their bodies 
with pollen-dust, and rub them accidentally against the pistils 
of other flowers. When the pollen falls on the stigma, it is 
caused to adhere to it by a honey-like secretion from its surface; 
and after a short time it undergoes a remarkable change which 
closely resembles that already described in the spore of the Cryp- 
togamia. 

438. The outer coat of the pollen-cell appears to burst at one 
or two points, and to allow the inner coat to pass out through it 
in the form of a tube. This tube insinuates itself between the 
cells of the stigma, and passes down between the long and 
loosely-arranged cells of the style. It gradually extends, until 



276 



FERTILIZATION OF THE OVULES. 





Fig. 51. 
Section of the top of 
the style of Snap. dra- 
gon ; showing the pas- 
sage of the pollen- 
tubes between its 
cells. 



Fig. 52. Pollen-grain, of Oeno- 
thera biennis, sending its tubes 
c, a, between the cells, b t of the 
stigma. 

it reaches the ovarium itself, even 
when the style is several inches 
long. The pollen-grains are not 
always globular, but are some- 
times triangular, and emit a pol- 
len-tube at each corner, as in the 
accompanying figure ; such are 
analogous to the spores of Mosses, 
which put forth several tubes. 
The tubes, when they arrive at the ovarium, direct themselves 
towards its different chambers, and have been seen to enter the 
apertures in the several ovules, which are at that time directed 
towards the part of the base of the style, from which the pollen- 
tubes project themselves. Sometimes a considerable change in 
the position of the ovule is necessary, in order that the foramen 
should be applied to the right portion of the wall of the ovary ; 
but this change always takes place just as the pollen-tubes are 
passing down the style. The granules which the pollen-grain 
originally contained, are seen to pass down the tube ; and some 
of them are conveyed, by it into each ovule. Whilst yet within 
the tube, they are seen to develope themselves into new cells ; 
and these cells are the rudiment of the future plant. 

439. The germs are thus conveyed into a sort of receptacle 
where they are supplied with nourishment that has been 
previously prepared and stored up for their use by the parent 



EA.RLY DEVELOPMENT OF THE EMBRYO. 277 

structure ; and they are thus greatly assisted in their early deve- 
lopment. The pulpy matter contained in the ovules consists of 
starch and sugar ; and these nutritious substances are absorbed 
by the cells of the embryo, which increase at their expense. The 
first increase of these cells does not so much tend, however, to 
form those parts which are afterwards to be developed into the 
stem, root, and leaves ; as to produce those temporary structures, 
termed cotyledons or seed-leaves (h 21,) which are destined like 
the primary frond of the Ferns, to assist for a time in the deve- 
lopment of the permanent structure, and then to wither and de- 
cay. Hence, at the time of the ripening of the seed, the cotyledon 
(which is sometimes double, sometimes single — see \ 440-2) 
forms the greatest part of the embryo or young plant. Besides 
this, the seed contains a considerable quantity of starch, destined 
for the nourishment of the young plant, when it is beginning to 
sprout, and whilst yet unable to take in food for itself. This 
starch is sometimes absorbed into the tissue of the cotyledons, 
rendering them thick and fleshy, as in the Pea or Bean ; and 
then these, with the small germ to which they belong, form the 
entire contents of the seed. In other instances, however, the 
cotyledons are thin leafy organs, and occupy, with the germ, 
but a small part of the seed ; the remainder then consists of a 
separate store, which closely resembles the yolk-bag of the egg, 
and is termed the albumen. This is the case in the seeds of all 
Mono-cotyledonous plants, and also in some Dicotyledons, as 
the Ash and Horse-chestnut. 

440. The structure of the seed of 
the two principal divisions of the Pha- 
nerogamia is shown in the adjoining 
figures. In Fig. 53, is seen that of the 
Bean, a Dicotyledon, after the seed- 
coats have been stripped off, and the 
cotyledons separated. The two large 
p. i« ~" fleshy lobes, a, a, are the cotyledons. 

Seed of the Bean, with its into which the whole of the starch 
cotyledons, a, a, separated; originally contained in the ovule has 




278 



DICOTYLEDONOUS SEED. GERMINATION. 



been absorbed. Between these is the real germ ; the upper ex- 
tremity of which, termed the plumula, subsequently developes 
itself into the stem, and puts forth leaves ; whilst the lower part, 
which is always directed towards the foramen, becomes the root. 
The plumula sometimes presents the appearance of the plant in 
miniature ; its leaves and buds being quite discernible, though on 
a very small scale. The subsequent development of the germ 
contained in the seed into the perfect plant, is that which in its 
early stage is known as germination. Of the 
causes which excite it, we shall presently 
speak. When a seed like that of the Bean be- 
gins to germinate, it first swells and bursts its 
seed-coats; the plumula then extends up- 
wards, bringing the cotyledons just above the 
surface of the ground ; whilst the radicle pe- 
netrates it in the opposite direction. In some 
plants, however, the cotyledons remain under 
ground, as in the Oak ; and there are a few 
in which they are entirely absent. The co- 
tyledons, when exposed to the light, become 
green ; and perform for a time (though im- 
perfectly) the functions of leaves, at the same 
time yielding to the young plant the nourish- 
ment they contain. By the time this is ex- 
-p- hausted, the true leaves and roots are 

Germination of Dico- sufficiently developed for the support of the 
tyledonous seed; a , P lu- structure . and the cotyledons, being then no 
mula; b, radicle, c, c, / ' & 

cotyledons. longer required, decay away. Thus it is 

seen that, in all the essential points, the history of the young Pha- 
nerogamic plant corresponds exactly with that of the young 
Fern ; — the chief difference consisting in this ; — that the develop- 
ment of the former, up to the time when its cotyledon or primary 
frond ceased to support it, is assisted by the nourishment pre- 
pared for it by the parent ; whilst the latter has no such assist- 
ance, but obtains its nourishment from the surrounding air and 
moisture. 




STRUCTURE OP THE SEED. 



279 




Fig. 55. 
Seed of the Marvel of Peru. 



441. The adjoining repre- 
sentation of the seed of the 
Marvel of Peru affords an 
example of a dicotyledonous 
seed possessing leafy cotyle- 
dons and a separate albumen ; 
in these, the process of germi- 
nation is the same, except that 
the cotyledons only perform 
the functions o£ temporary 
leaves, the nutritious part of the seed being retained within its 
coats, until it is exhausted by the young plant. 

442.. In the seeds of the 
Monocotyledons, the struc- 
ture of which is illustrated by 
the accompanying figure of 
that of the Onion or Lily, the 
albumen is always separate ; 
and the embryo, which occu- 
Fig. 56. Sections of SeedTf Onion ; Pies but a small proportion of 
a, a, albumen ; 6, 6, embryo. the whole mass, cannot al- 

ways be readily distinguished in the midst of it, until germination 
commences. The cotyledon at first completely sheathes the plu- 
mula, which afterwards pierces it, and unrolls its first true leaf. 

443. Now it is an interesting fact, that the division of the 
Phanerogamia, founded upon the structure of the seed, exactly 
corresponds with that formed according to the structure of the 
stem ; — that is, all Exogens are Dicotyledonous (with only a few 
apparent exceptions;) and all Endogens are Monocotyledonous. 
Moreover, all the Acrogens, which have no regular method of 
adding to the diameter of their stem, are destitute of the power 
of forming true seeds, — the germs being, as it were, at once cast 
upon the world, instead of being reared and cherished by parental 
care. It has formerly been pointed out that Exogens, Endogens, 
and Acrogens differ also in the distribution of the veins in their 
leaves (§ 229 — 32 ;) and it may here be mentioned that they differ 





280 



CONDITIONS OF GERMINATION. 



also in the number of parts of which the flower is usually com- 
posed. Thus, in Exogens, the regular number of stamens is 
either four or five, or a multiple of one of these numbers ; and 
that of the carpels is similar; whilst in Endogens the number of 
the same parts is three, or a multiple of it. The number of the 
external or leafy parts of the flower follows the same laws, as 
will be hereafter explained. 

444. The conditions requisite for the germination of the seed, 
are warmth, moisture, and the presence of oxygen. The process 
is also favoured by darkness. The influence of each of these 
agents will be readily understood. No vital action can go on 
without a certain amount of heat; and where this is not pro- 
duced within the being, it must be derived from without. The 
germination of the seed is as much dependent upon warmth, 
therefore, as the hatching of the egg of a bird ; though the amount 
it requires is not nearly so great. Moisture is also evidently 
required, for the conversion into a fluid state of the dry nutriment 
which has been previously stored up in the seed ; and no change 
can commence until this be supplied. The presence of oxygen is 
necessary, because the conversion of starch into sugar requires (as 
formerly stated \ 283,) that some of the carbon of the former should 
be set free; and this can only be accomplished by the union of it 
with oxygen, so as to form carbonic acid. This process is favoured 
by darkness, because light has a tendency to produce the contrary 
change — the fixation of the carbon within the structure Q 286.) 

445. It is interesting to observe how all these conditions are 
supplied, in the ordinary course of Nature, by the soil in which 
the seed is dropped. If it be sown during the spring or summer, 
it speedily begins to germinate ; but if it is deposited in the 
autumn, it remains almost unchanged, until the winter has passed, 
and the returning warmth of the air and earth arouses it into 
activity. It is seldom that the soil is so completely destitute of 
moisture, for any long time together, as not to be able to excite 
seeds to germinate ; but their sprouting is well known to be 
favoured by damp weather; and if seeds, through being put into 



CONDITIONS OF GERMINATION. 28 1 

the ground during a drought, remain undeveloped, they are brought 
forwards very rapidly by a genial shower. A porous soil is to be 
preferred on account of the free admission of air which it gives to 
a germinating seed, as well as for the other processes of vegeta- 
tion (§ 178, 9.) A stiff clay soil prevents this necessary contact ; 
and thus impedes germination. So complete a check, indeed, 
may be thus produced, that it has been proposed to bury seeds in 
clay rammed hard, when it is desired to convey them from one 
part of the world to another through very hot climates ; the high 
temperature of which might destroy their vitality, if its influence 
were not partly prevented by the bad-conducting power of the 
mass in which they are thus enclosed. If seeds be buried very 
deep, even in a light soil, the contact of oxygen will be suffi- 
ciently impeded to prevent their germination ; and the bringing 
such seeds nearer to the surface will then have as much influence 
in causing them to sprout, as the supply of either of the agents 
just mentioned, which might have been previously deficient. 

446. The seeds of most plants are endowed with a remarkable 
power of preserving their vitality for an almost unlimited time, if 
they are placed in circumstances which neither call their properties 
into active exercise, nor occasion the decay of their structure. 
The conditions most favourable for this preservation will evidently 
be a low or moderate temperature, dryness of the surrounding 
medium, and the absence of oxygen. If all these be supplied in 
the most favourable manner, there seems no limit to the period 
during which seeds may retain their vitality, — that is, their power 
of performing their vital operations, when placed in the proper 
circumstances. And even if moisture or oxygen be not entirely 
excluded, the same effect may result, provided that the tempera- 
ture be low and uniform. Thus the seeds of most plants may be 
kept for several years, freely exposed to the air, provided they 
are not exposed to dampness, which will either cause them to 
germinate or to decay. Some of those which had been kept in 
seed-vessels of plants preserved in the herbarium of Tournefort, 
a French Botanist, were found to retain their fertility after the 
lapse of nearly a century. 



282 



PROLONGED VITALITY OF SEEDS. 



447. Instances are of no unfrequent occurrence, in which ground, 
that has been turned up, spontaneously produces plants different 
from any in their neighbourhood. There is no doubt that in some 
of these cases, the seed is conveyed by the wind, and becomes de- 
veloped only in spots which afford it congenial soil, as was formerly 
mentioned in regard to the spores of the Fungi (§ 50.) Thus, it 
is commonly observed that clover is ready to spring up on soils 
which have been rendered alkaline by the strewing of wood-ashes 
or the burning of weeds, or which have had the surface broken and 
mixed with lime. But there are many authentic facts which can 
only be explained upon the supposition that the seeds of the newly- 
appearing plants have lain for a long period imbedded in the soil, 
at such a distance from the surface as to prevent the recess of 
air and moisture ; and that, retaining their vitality under these 
conditions, they have been excited to germination by exposure to 
the atmosphere. The following possesses considerable interest. 

448. To the westward of Stirling, there is a large peat-bog, a 
great part of which has been flooded away, by raising water from 
the river Teith, and discharging it into the Forth,— the object of 
this process being to lay bare the under-soil of clay, which is then 
cultivated. The clergyman of the parish was on one occasion 
standing by, while the workmen were forming a ditch in this 
clay, in a part which had been covered with fourteen feet of peat 
earth ; observing some seeds in the clay which was thrown out 
of this ditch, he took them up and sowed them; they germinated, 
and produced a species of Chrysanthemum. A very long period 
of years must have probably elapsed, whilst the seeds were get- 
ting their covering of clay ; and of the time necessary to produce 
14 feet of peat-earth above this, it is scarcely possible to form an 
idea, but it must have been (in the natural course of things) 
extremely great. 

449. The following circumstance, which occurred about 30 years 
ago in the State of Maine, in North America, is, perhaps, still more 
remarkable. Some well-diggers, when sinking a well, at the 
distance of about 40 miles from the sea, struck, at the depth of 
about 20 feet, a layer of sand ; this strongly excited curiosity and 



PROLONGED VITALITY OF SEEDS. 



283 



interest from the circumstance that no similar sand was to be 
found any where in the neighbourhood, or any where nearer than 
the sea-beach. As it was drawn up from the well, it was placed 
in a pile by itself; an unwillingness having been felt to mix it 
with the stones and gravel which were also drawn up. But when 
the work was about to be finished, and the pile of stones and 
gravel to be removed, it was found necessary to remove also the 
sand-heap. This, therefore, was scattered about the spot on 
which it had been formed, and was for some time scarcely re- 
membered. In a year or two, however, it was perceived that a 
great number of small trees had sprung from the ground over 
which the sand had been strewn. These trees became, in their 
turn objects of strong interest ; and care was taken that no injury 
should come to them. At length it was ascertained that they 
were Beach-Plumb trees ; and they actually bore the Beach-Plumb, 
which had never before been seen, except immediately upon the 
sea-shore. Theset rees must, therefore, haves prung up from 
seeds which had existed in the stratum of sea-sand pierced by the 
well-diggers ; and until this was dispersed, in such a manner as to 
expose them to the air, they remained inactive. " By what con- 
vulsion of the elements," adds the narrator, " they had been 
thrown there, or how long they had quietly slept beneath the 
surface of the earth, must be determined by those who know very 
much more than I do." 

450. The following is an example of the same general fact, 
which is interesting from its connexion with historical events. 
In the year 1715, during the rebellion in Scotland, a camp was 
formed in the King's Park (a piece of ground belonging to the 
castle) at Stirling. Wherever the ground was broken, broom 
sprang up, although none had ever been known to grow there. 
The plant was subsequently destroyed ; but in 1745, a similar 
growth appeared, after the ground had been again broken up for 
a like purpose. Some time afterwards, the Park was ploughed 
up, and the broom became generally spread over it. The same 
thing happened in a field in the neighbourhood, from the whole 
surface of which about nine inches of soil had been removed. 
The broom-seeds could not have been conveyed by the wind, 



284 



PROLONGED VITALITY OF SEEDS. 



although the plant is a common one in the neighbourhood, be- 
cause they are heavy and without wings (§ 471. ;) and the form 
of the ground is such that no stream of water could have trans- 
ported them, or have covered them afterwards with soil. Such an 
effect must have resulted from the operation of causes continued 
through a long period of time. 

451. Perhaps the most remarkable instance on record, as 
presenting satisfactory proof of the lapse of at least 1600 or 
1 700 years, is one related by Dr. Lindley. " I have now before 
me," he says " three plants of Raspberries, which have been 
raised in the gardens of the Horticultural Society, from seeds 
taken from the stomach of a man, whose skeleton was found 30 
feet below the surface of the earth, at the bottom of a barrow,* 
which was opened near Dorchester. He had been buried with 
some coins of the emperor Hadrian." Corn-grains enclosed in 
the bandages which envelope the mummies, are said to have 
occasionally germinated, though most of them seem to have lost 
their vitality. There is nothing improbable in the fact ; but as 
the Arabs, from whom the mummies are commonly obtained, 
are in the habit of previously unrolling them in search of coins, 
&c. it is not always certain that the seeds which have sprouted 
were really at first enclosed with the mummies. 

452. When a plant is raised from seed, it will always bear 
a strong likeness to its parent ; and if the species be one which 
has little tendency to variation, it will resemble it very closely. 
But there are many species, which have a great disposition to 
present deviations from what may be considered their original 
form (§ 13-16 ;) and thus, from the seeds of the same parent it is 
often possible to produce, by a difference of treatment, a number 
of plants differing considerably from one another. Whatever such 

* These barrows, as they are termed, are large mounds of earth, which 
are very common on the downs along the south coast of England. They are 
evidently artificial, not natural; and when dug into, are usually found to 
contain human remains, with pottery ; weapons, &c. Hence they are 
evidently burial-places ; and as a large number of them are generally found 
together, they seem to have been erected on fields of battle, to contain the 
bodies of the slain. 



HYBRID PLANTS. 285 

differences may be, however, these plants are all regarded as be- 
longing to the same species, since they are descended from a 
common stock ; and by such experiments, it is often possible to 
show that plants which have been considered as distinct species, 
have no real title to be so classed (§ 16.) 

453. It is often possible, however, to produce seeds capable 
of giving origin to plants that shall combine the characters of 
two different races. This is done by placing the pollen of one 
species upon the stigma of another ; so that the germ, furnished 
by one, shall be nursed (as it were) by the other. It is not diffi- 
cult to understand how the germ thus influenced, should be sub- 
sequently developed into a form differing from that of its own 
parent ; for the germs of Cryptogamia, which are not received 
into any ovule, but are dependent upon the elements alone for 
their support, are often developed (especially among the lower 
tribes) into forms very different from that which they would 
naturally present. Thus a Mucor, a sort of Fungus concerned 
in the production of mouldiness, has been seen growing in water 
in a form so like that of a Conferva, that it was only recognised 
as a Fungus when it lifted up its fructification above the fluid. 

454. The plant developed from a seed produced by the agency 
of two races, is termed a hybrid. It is necessary, in order that 
the seeds thus formed should be fertile, that the parent species 
should be nearly allied to each other; and it is very seldom that 
a hybrid can be produced when they do not belong to the same 
genus. Now, if the hybrid bear flowers, and its stigma be fer- 
tilized with its own pollen, it may produce seeds that can be 
raised into plants like itself; and these may flower and produce 
a third generation in like manner. But there is no instance in 
which a hybrid race, which has thus originated in the intermixture 
of two species really distinct, has ever been continued without 
intermixture beyond the fourth or fifth generation. The plant, 
when not fertile by itself, may boar seed, if its stigma be sprinkled 
with the pollen of one of its parent species; and its pollen may be 
fertile when placed on the stigma of either of these. In this man- 
ner, a race intermediate between the hybrid and one of the parent 

Q5 



286 LIMITS OF HYBRIDITY. STRUCTURE OF FLOWER. 

species is produced ; and this is continued longer, just in propor- 
tion as it is caused to approach the pure breed, by a successive 
intermixture of this kind. The end of all hybrid races, produced 
between species really distinct, appears to be, therefore, that — 
either the race becomes soon extinct, which it will do if kept 
separate, — or it merges into one of the parent races, if continued 
by intermixture with either of them. This principle affords a 
valuable test for determining what really are, and what are not, 
distinct species ; for if a hybrid race can be produced between 
them, which continues to be fertile of itself, the probability is 
strong that they are only varieties. Cultivators of flowers are 
constantly in the habit of producing such new races between the 
different varieties of many plants, — for instance, the South 
American Amaryllis and the Calceolaria; both these species are 
very much disposed to spontaneous variation; and, by selecting 
the most beautiful of the new races that spontaneously originate 
from their seeds, and causing these to produce hybrids, a still 
larger amount of variety, both in form and colour may be obtained. 
These hybrids are of equal fertility with their parents, since the 
latter are not separated by any really essential difference. 



455. Having now considered the general structure and offices 
of those organs of Phanerogamia, which are most interesting to 
the Physiologist, from their connexion with the important function 
of Reproduction, we shall notice those parts of the flower which 
are less essential to this object, but which commonly excite more 
interest, on account of the varied and beautiful forms and colours 
they present ; — namely, its external leafy portion. This may be 
altogether termed the perianth, or floral envelope ; the essential 
portion of the flower being, as before explained, the stamens and 
pistil occupying its centre, which are sometimes destitute of any 
protection. The perianth may be regarded, as consisting, in its 
most regular form, of two circles (arranged like the whorls or 
verticils of leaves) of leafy organs ; of which the outer circle is 



CALYX AND COROLLA. 287 

generally green, and the inner one coloured.* Of this outer 
circle, the leaves or sepals not unfrequently grow together, or ad- 
here, at their edges; so that a sort of cup is formed; hence the 
whole is termed the calyx (cup.) The inner whorl is termed the 
corolla, and its divisions are called petals ; they not unfrequently 
grow together in the same manner (as in the Campanula or 
Harebell) forming a second cup within the calyx. 

456. When the calyx seems formed of but one piece, in con- 
sequence of the adhesion of its leafy portions, it is said to be mono- 
sepalous (possessing but a single sepal ;) and when the petals have 
united in a similar manner, the corolla is said to be monopetalovs. 
Though these terms are not strictly correct (since there are really 
as many sepals and petals in the one case as in the other,) they 
are convenient, and are often employed in describing plants. 
The real nature of such a calyx or corolla is shown by varieties, 
or monstrosities, like that delineated in the adjoining figure ; here 

the regular form of a mono- 
petalous corolla, (in which 
the petals have grown to- 
gether to form a tube, and are 
only separate at the top) is 
shown at a ; whilst b shows 
the separate condition of the 
Fig, 57. a, monopetalous corolla ; b, petals which is occasionally 
monstrous form of the same. seen Qg the consequence of a 

want of adhesion between their edges. Different kinds of flow- 
ers, too, exhibit every variety between the completely-separate 
and the completely-adherent condition of the sepals and petals : 
and these differences are often very useful in distinguishing them 
from each other. 

457. Outside the calyx is not unfrequently to be found another 
whorl of leafy bodies, more resembling in their aspect the ordinary 
leaves of the plant; these are called bracts, and are well seen in 

* In Botanical language, the term coloured always moans that the pari 
is not green, green being regarded as no colour in Plants, A white dower 
is spoken of as coloured. 




288 BRACTS. ANALOGY OF SEPALS AND LEAVES. 

the Strawberry, where they surround and alternate with the 
sepals of the calyx. When no complete circle of them is seen, one 
or two are often present, and then they are generally larger. 
They do not always immediately surround the flower ; but are 
often to be found at the bottom of the flower-stalk. These bracts 
may be regarded as establishing the transition of form and struc- 
ture between the common leaves of the plant, and those modified 
or metamorphosed leaves which form the perianth. Sometimes 
they can scarcely be distinguished from the former; whilst in other 
cases, they are brightly coloured, and more closely resemble the 
latter ; and, in the Hydrangea and some other plants, they really 
constitute the most showy portion of the flower, being very large 
and brilliant, whilst the flower they enclose is so small as to be 
almost overlooked. In many instances, the bracts form so gra- 
dual a transition between the true leaves and the parts of the 
flower, that it is very difficult to say where the former end and 
the latter begins. This is the case in the double Pseony, — a plant 
now very common in gardens. Its lower leaves are very com- 
plex in their structure, being divided into a great number of seg- 
ments {\ 235 ;) in tracing them up the stem, they are found to be- 
come simpler and simpler in their character as they approach the 
flower, and also to diminish in size; and at the same time their 
spiral arrangement round the stem becomes more evident, the in- 
tervals between them being diminished. In this manner, they may 
be at last traced into the outermost whorl of the leafy parts com- 
posing the flower ; and it is quite impossible to specify the exact 
place at which the true leaves may be said to end, or the calyx 
to commence. 

458. From this it would appear that there is no essential dif- 
ference between the sepals of the calyx and regular leaves ; and 
examination of their structure bears out the conclusion. If we 
take an example from a plant in which the sepals are distinct from 
each other, and green, we should find it difficult to assign any im- 
portant characters in which they differ from leaves. They possess 
two layers of cuticle, furnished with stomata; having green cellular 
tissue or parenchyma between them, supported by veins consist- 
ing of woody fibre and vessels. There are many cases, however, in 



ANALOGY OF SEPALS AND PETALS. 289 

which the calyx is brightly coloured, equalling the corolla in 
beauty, and even surpassing it in brilliancy. In the Lilies and 
Tulips, we find the perianth composed of six coloured parts, 
which seem to spring at once from the flower-stalk, without bracts 
or calyx. But, if they be examined, it will be found that three 
of these arise lower down than the others, and therefore partly 
enclose them; so that these three are to be regarded (in spite of 
their colour) as sepals of the calyx ; and it may often be observed 
that, though coloured in their interior, they are greenish outside, 
especially along their middle. In the Fuchsia, a plant which is 
now becoming naturalized in our gardens, though formerly con- 
sidered a rare exotic, the calyx is even more brightly coloured 
than the corolla. This change of colour, however, by no means 
disproves what has been said of the analogy between sepals and 
leaves ; since, as formerly noticed, leaves themselves occasionally 
undergo similar changes, and the colouring principle seems to 
consist in all cases of nearly the same substance, in different 
states of chemical combination (§ 386.) Farther, the calyx not 
unfrequently returns to the form of true leaves, in flowers in 
which its regular appearance is very different ; such irregular 
formations, which are termed monstrosities, are in this, as in 
many other instances, very instructive to the physiologist, in 
leading him to the knowledge of the true character of organs, of 
which the external form may have been greatly changed. 

459. Similar remarks may be made upon the real nature of the 
petals of the corolla. They are almost always coloured ; but they 
still preserve their leafy structure, having cuticle, stomata, paren- 
chyma, and veins. It has been seen that, in the Tulip and Lily 
tribe, there is no essential difference between the sepals and 
petals; what is true of the former, therefore, must be also true of 
the latter. Farther, in the Foeoivy, the transition from the form of 
the sepal to that of the petal, is as gradual as that from the ordi- 
nary leaf to the sepal. [[ we trace the portions of the perianth 
from without inwards, we may observe that the green leafy sepals 
are slowly changed, — in the first place by having their points and 
edges turned from green to pink, and becoming more delicate in 

OR* 



290 



TRANSFORMATION OF PETALS AND STAMENS. 



their structure ; — next the inner side is seen to be completely 
coloured, while the back is still greenish in its centre ; — and finally 
the whole is converted into an ordinary petal. But even where 
the appearance of the petals is the farthest removed from that of 
ordinary leaves, it is very common to find monstrosities which 
show that there is no essential difference. The common Wood- 
Anemone, for example, not unfrequently presents several varieties 
in the character of the sepals and petals, intermediate between 
what may be regarded as natural to them, and that of the ordi- 
nary leaves. Thus, the calyx may be converted into a whorl of 
true leaves, whilst the white petals have become green and 
resemble the ordinary sepals ; or the metamorphosis may have 
proceeded farther, and the petals as well as the sepals may have 
been converted into ordinary leaves. 

460. The structure, appearance, and functions of the stamens 
are so different from those of the parts of the perianth, that it 
would scarcely appear probable that they too are transformed 
leaves; and yet this will prove to be the case. There are many 
flowers in which the transition from the form of the petal to that 

of the stamen, is as gra- 
A /FN dual as those already de- 
frlVi (\\\\ scribed. This is the case, 
for example, in the Pae- 
ony ; and it is still more 
evident in the common 
White Water Lily, the 
Fig-. 58. Stages of transformation of petals principal stages of trans- 
of White Water Lily into stamens. formation in which are 

represented in the adjoining figure. The petal is first thickened 
near its point, by a deposite of yellow substance, which, when 
examined, is found to be pollen. This thickened part gains upon 
the expanded portion of the petal, which becomes contracted in 
a corresponding degree, as we advance nearer the centre of the 
flower ; until we arrive at the regular form of the stamen, in 
which we observe that the two thickened parts have met as 
anther-lobes, and that the leafy portion of the petal is contracted 




TRANSFORMATION OF STAMENS. 291 

into the filament supporting them. The inner rows of stamens 
(of which there are several) are still more contracted, not being 
fully developed ; and here we lose all trace of the leafy appearance. 

461. Although the usual appearance of the stamens is such 
as was formerly described, there are several flowers in which they 
ordinarily have very broad expanded filaments ; and these organs 
are subject to the same kind of transformation into the leafy cha- 
racter as are the portions of the calyx and corolla. The trans- 
formation of stamens into petals, in fact, is extremely common ; 
it being generally in this manner that double flowers are pro- 
duced from single ones. In the wild Rose, for example, we find 
but a single row of petals, surrounding a very large number of 
stamens; whilst in the cultivated Rose of gardens, there are 
several rows of petals, and the number of stamens has proportion- 
ally diminished. The Rose is a flower which is very liable to 
produce monstrosities or irregular growths ; and it is not uncom- 
mon to find this transformation more complete, — the stamens, as 
well as the petals and sepals, being converted into true leaves, so 
that the flower is entirely green. The same is often the case 
with the Wood- Anemone. No farther evidence, then, is required 
to prove that the elements of the leaf and the stamen must be 
the same (although their fully-developed forms are so different ;) 
and that that these elements may be developed into one form or 
the other according to circumstances, with which we are as yet 
only in part acquainted. 

462. We now come to the Pistil, which occupies, as formerly 
stated, the centre of the flower. In considering its real nature, 
it is always necessary to regard it as made up of a number of 
separate carpels (§ 434,) whether or not they can be completely 
distinguished ; — just as the monosepalous calyx and the mono- 
petalous corolla are considered as formed by the adhesion of 
their several constituent portions. We have to examine 
then, what is the real character of each carpel; and this is 
sometimes manifested to us in a remarkable manner. When 
the carpels are distinct, and are fully developed, they not 
unfrequently present a very leafy appearance. Thus, the pod 



292 



TRANSFORMATION OF CARPELS, 




of the Pea, when 
opened, is seen 
not to differ 
essentially from 
what a leaf with 
its two edges 
rolled together 
would be; the 
prolongation of 
the stalk corre- 
sponds with the 
midrib, and the 
Fig. 59. two valves of 

the pod are the two lobes of the leaf Instances occasionally pre- 
sent themselves in which this is seen more decidedly, from the want 
of devolopment of the ovules, and the non-closure of the pod, so 
that its leafy aspect is less departed from. There are little pro- 
jections, however, from the thickened edges of this carpellary leaf, 
which show where the ovules should have been. A still more 
interesting monstrosity is almost constantly presented by the 
double cherry. The centre of the flower is occupied by a small 

leaf in place of the usual carpel. 
This leaf (Fig. 60 ; a) has the two 
edges folded towards each other, 
and the midrib is greatly pro- 
longed, having a little dilatation at 
its summit. If this be compared 
with the carpel of the cherry, seen 
at c, no doubt can be entertained 
that the two sides of the leaf answer 
to the walls of the ovary, the pro- 
longed midrib to the style, and its 
dilated extremity to the stigma. In 
some instances the flower contains 
two such leaves ; and they are then always seen to present their 
hollowed faces towards each other, in the manner seen at b. This 




Fisr. 60. 



REGULAR ARRANGEMENT OF THE PARTS OF FLOWERS. 293 

precisely corresponds with the position of the true carpels, 
shown at d; in which the suture, or line of junction of the two 
edges, of each carpel is opposite to that of the other. If any 
farther proof were required, of the carpel being a transformed 
leaf, it is afforded by the fact that, in Roses, Anemonies, Ranun- 
culuses, and other such flowers, which are liable to have their sta- 
mens converted into petals, or into true leaves, the carpels not un- 
frequently undergo the same changes, so that the whole flower is 
metamorphosed into a bunch of leaves, which are still arranged, 
however, on exactly the same plan with the parts of the real 
flower. 

463. The usual arrangement of these parts corresponds pre- 
cisely with what was formerly stated of the disposition of the 
leaves (§ 185.) When the spiral, which may be regarded as their 
regular mode of arrangement, is converted into a whorl or ver- 
ticle, by the non-development of the intervening part of the axis, 
and two or more of these whorls succeed one another, their seve- 
ral leaves do not correspond in the direction in which they issue 
from the stem, but are so placed that the leaves of each are above 
or below the intervals between the leaves of the other. When 
this is the case, the whorls are said to alternate with each other. 
Now the regular flower may be considered as made up of five 
such whorls, arising from nearly the same part of the axis ; and 
they are disposed alternately with each other. Thus the sepals of 
the calyx alternate with the bracts ; the petals of the corolla alter- 
nate with the sepals, and are opposite to the bracts ; the stamens 
alternate with the petals, and are opposite to the sepals ; and the 
carpels alternate with the stamens and are opposite to the 
petals. 

464. This very simple law, regulating the position of the 
parts of the flower, is apparently subject, however, to many ex- 
ceptions ; but these all arise from the interference of other 
causes. For example, the number of parts may be so much 
increased, that they cannot be all arranged in one whorl, and 
they then Conn additional verticils; which, however, still fol- 
low the same principle of arrangement For example, 



294 



ARRANGEMENT OF THE PARTS OF FLOWERS. 




Fig. 61. 
Plans of flowers; a, Cherry 



the adjoining figure 
shows a plant (a) of the 
flower of a Cherry; in 
the outer circle are 
marked the places of 
the five sepals, and in 
the next those of the 
five petals which alter- 
' qui ' nate with them. Within 
these, however, we find no less than twenty stamens ; but these 
may be regarded as composing four whorls with five in each, 
apparently blended together, however, by the closeness of their 
origin. The other diagram (b) is the plan of the flower of a 
Squill, in which, as in other Endogens, the parts are disposed in 
threes not in fives. The outer circle has three dots indicating the 
places of the three sepals ; and on the inner one the petals are 
indicated in like manner, and are seen to alternate with the 
former ; the stamens are six in number, and distinctly form two 
rows, of which the outer one is opposite to the whorl of the calyx, 
and the inner one to that of the corolla ; and with this, again, the 
carpels would alternate. 

465. An apparent irregularity, however, is more frequently 

produced by the absence 
of some of the parts. 
Thus, in the Primrose, 
there are five sepals, five 
petals, and five stamens; 
, but the stamens are op- 
Y\„ 62. posite to the petals, in- 

Plans of flowers; a, Primrose ; 6, Samolus. stead of alternating with 
them (Fig. 62 ; a.) Now the explanation which the Botanist 
would offer of this irregularity, is, — that there must be a row of 
stamens intermediate between the petals and the stamens, which 
from some cause have not been developed. And this is found to 
be really the case ; for in the Samolus, a plant otherwise formed 
upon the same plan as the primrose, five little scales, which are 
partly-developed stamens, appear in the situation of the absent 




ARRANGEMENT OF THE PARTS OF FLOWERS. 



295 




Fig 63. a, plan of flower of Sage; 6, de- 
velopment of stamens in allied genus. 



row. In the Sage, again, 
we find a calyx of five 
sepals, and a corolla of 
five petals ; but only two 
stamens are seen within 
(Fig. 63 ; a.) Now, upon 
looking attentively at the 
inside of the tube of the 
corolla, two little scales are often to be seen growing in the place 
where two of the deficient stamens should have been, — that is, 
alternating with the petals; these two scales are often developed 
as perfect stamens, in flowers which are otherwise constructed 
exactly like the Sage (b) ; and even the fifth makes its appearance 
in some instances, exactly where it should regularly be found. 
Such deficiencies are often to be noticed; thus in the genus 

Bauhinia, which has, pro- 
perly ten stamens arranged 
in two whorls, there are 
some species in which only 
three, or even but one, per- 
fect stamen is developed. 
Deficiency in the number 
Fig. 64. Plans of Flowers of Bauhinia. of Carpels in the pistil is 

even more common; and it is in fact rare to find a flower 
which presents a structure that may be considered perfectly re- 
gular, as well in its form as in the number of its parts. With- 
out forming some such standard, however, it would be impos- 
sible to obtain a definite idea of the nature of the deviations, of 
which some of the principal kinds will have to be considered 
hereafter. 

466. One of the commonest of these deviations is that in which 
the calyx appears to arise, not, as is usual, below the ovarium, 
but above it. In this case it will be found that the real position 
of the parts is the same ; but that the perianth wraps round or 
encloses the ovarium and spreads itself out only when freed from 
it. The stamens too, not unfreqnentlv seem to arise from the 




296 VARIETIES OF STRUCTURE. — DISK. 

corolla, instead of from the axis of the flower ; but this effect is 
produced in a similar manner — namely, by their adhesion at their 
lower part to the inner side of the petals. The stamens, again, 
sometimes adhere to each other, so as to form a complete tube, 
surrounding the pistil, 

468. In the foregoing instances, the symmetry of the flower 
is not destroyed ; that is, it may be divided into two similar 
halves by a line crossing it in any direction. But there are many 
irregularities resulting from the unequal development of the dif- 
ferent parts of the same whorl, and from the adhesion of these 
parts to each other in various ways ; so that the whole form of 
the flower sometimes appears completely changed, and there is 
only one direction in which it can be divided into two equal 
halves. This is the case in the flower of the Pea or Bean, for 
example ; in which, as in other plants of the Papilionaceous group 
(so named from the resemblance of the flower to a Papilio or But- 
terfly,) there is one broad petal standing erect, two separate ones 
termed the wings, which are prolonged from its base, and two 
others united together, forming what is termed the keel, which is 
enclosed between the last. 

468. The flower is usually placed at the end of the flower- 
stem, or of its subdivision of it ; and the tendency in this stem to 
lengthen, appears to be checked by the development of a flower- 
bud. It commonly swells out at the insertion of the perianth, 
forming what is called the disk or receptacle; and in this, as for- 
merly stated, nourishment is frequently laid up, in the form of 
starch, for the development of the young ovules (§ 285.) This re- 
ceptacle sometimes grows upwards between the carpels, and even 
encloses them. In other cases, it extends so much as to separate 
the carpels from one another ; this is the case in the Strawberry, of 
which the fruit is the swollen receptacle, whilst the little bodies 
scattered over its surface (commonly termed seeds) are in reality 
the carpels. Sometimes, however, it happens that the flower- 
stem continues to grow between the points from which the solids 
proceed ; and they are then separated, from each other, just as 
are leaves in like circumstances (§ 304.) The spiral line, in which 



TRANSFORMATIONS OF THE ENTIRE FLOWER. 297 

the different parts of the flowers are inserted round the axis, then 
becomes very evident. This is the case not unfrequently in the 
Double Tulip ; and as the parts of the flower are generally at the 
same time more or less changed into the leafy character, the re- 
semblance of the whole flower to a leaf-bud or undeveloped branch 
then becomes very obvious. Sometimes after giving off the whorls 
of the perianth, the flower-stalk is prolonged through their centre, 
and bears another bud at its extremity ; this is by no means un- 
common in Roses. It is well known to Gardeners that, by a still 
farther change, flower-buds may be actually converted into leaf- 
buds, and developed into true leaf-bearing branches ; a fact which 
sufficiently proves that every part of the flower is formed out of 
the same elements with leaves, and that the development of either 
may take place according to circumstances. Hence we know why 
a difference in the amount of nutrition which the plant receives,, 
should influence its tendency to the production of flowers and fruit. 
It has been stated that, in each of the parts of the flower, there is 
a tendency to revert to the leafy form ; and this is especially the 
case with the stamens, which are often converted into petals (thus 
changing a single flower into a double one) when the plant is 
transferred from the poor soil in which it may be naturally grow- 
ing, into the rich mould of a garden. Now if a plant be over-sup- 
plied with nourishment, it will run to leaf, as it is termed, — that 
is, it will develope too many leaf-buds, and will not put forth flow- 
ers ; so that, in order to make it bear fruit, it is necessary to di- 
minish its quantity of sap ; one method of effecting which, is to 
dig a trench at a certain distance round the bottom of the trunk, 
so as to cut off part of the supply it receives from the roots. 

469. It might be objected to the statements here made, that 
the pollen and the ovules are so different from any thing which 
the leaf naturally produces, that no analogy can be imagined be- 
tween organs bearing these, and the ordinary leaves. But, if 
the structure of the pollen-grain be considered, it will be per- 
ceived to correspond precisely with that of other cells of Cellular 
tissue; differing chiefly in its power of separating itself from the 
rest, and of sending forth little granules which are to form new 
2G 



298 NATURE OF OVULES. FORMATION OF FRUIT. 

plants, instead of adding to the number of cells in the parent struc- 
ture. Every cell of the Confervae, it will be recollected (§ 424,) 
may be regarded as essentially a pollen-grain ; and therefore the 
difference cannot be really so great as it appears. Farther, in re- 
gard to the ovules, the fact heretofore mentioned (§ 240,) that cer- 
tain leaves have the power of producing little buds from their 
edges, becomes of great interest ; for, if the ovules could be re- 
garded as at all analogous to buds, it is evident that their situa- 
tion on the edges of the carpellary leaf would quite correspond 
with that of the buds of the Bryum, or of the Bog Orchis. And 
it has been proved by the occurrence of some curious monstrosi- 
ties, that this is a real analogy ; for a seed-vessel has been known 
to bear a set of little buds at the edges of its carpellary leaves, 
arranged just as the ovules should have been. 

470. We have in the last place to consider the structure of 
the Fruit, which is the mature or ripened ovary containing ferti- 
lized seeds. This frequently differs remarkably from the ovary 
which the centre of the flower contained, both in its external ap- 
pearance, and in the arrangement of its interior. For example, 
the Cherry, Plum, Almond, or other stone fruit, is formed by a 
remarkable change in the substance of the carpellary leaf; the in- 
ternal surface of this becomes hardened into the stone, whilst the 
external remains as a thin cuticle or skin ; and the pulp of the 
fruit is formed by the increase of the parenchyma or fleshy tissue 
of the leaf. Here each carpel originally contained several ovules, 
but only one of them is usually developed. In the ovary of the 
Chestnut, there are originally seven carpels or cells with two ovules 
in each, whilst the ripe fruit consists of but one cell and one seed ; 
so that no fewer than six cells and thirteen ovules are suppressed, 
in order to enable a single ovule to grow and be matured. It is 
not uncommon, however, to find two or even three chestnuts with- 
in a single shell, separated by slight partitions. The fruit of the 
Orange, as formerly mentioned, consists of the carpels, surrounded 
by the calyx which is adherent to their exterior, and having the 
space between their inner wall and the seeds they contain, filled up 
with a very succulent cellular tissue. On the other hand, in the 



STRUCTURE OF FRUIT. DISPERSION OF SEED. 299 

Apple, the carpels lie in the centre of the fruit, and their walls are 
somewhat horny ; the fleshy substance of the fruit is formed by the 
calyx, which is adherent to the exterior of the ovary ; and the 
parenchyma between its two surfaces swells out in ripening, in 
the same manner as does that of the carpellary leaf of the Plum. 
In the Medlar, the carpels have a hard or bony covering, and 
they lie separately in the midst of the pulpy envelope, which they 
acquire in like manner from the calyx. In the Strawberry, as 
formerly mentioned, the carpels are separated from each other 
by the receptacle, the expansion of which forms the fleshy part of 
the fruit. In the Raspberry, on the other hand, the receptacle is 
the white fleshy stalk which occupies the centre of the fruit ; and 
the pulpy portion consists of the carpels enclosing seeds. The 
pods of the Pea, Laburnum and other Leguminous plants, again, 
are single carpels, which sometimes grow to a great length, and 
contain many seeds. A great many more varieties might be 
enumerated ; but the mention of these will serve to give an idea 
of the mode in which the very curious transformation of the 
ovary into the fruit takes place. 

471. When the seeds are ripe and ready to be dispersed, the 
carpel usually splits either along the suture, or in the opposite 
direction, in order to set them free. There are many curious pro- 
visions for their dispersion to a great distance from the parent. 
Some of these, depending on the movements of the capsule, have 
already been explained. Many seeds are winged, that is, are 
furnished with a little expansion on each side, filled to catch the 
wind ; and thus they are wafted to places far distant from those 
in which they were produced. A very common provision is that 
of which the Dandelion seed is an example. This, as is well 
known, is furnished with a very light downy appendage, by which 
it is floated along with the slightest breath of air. Other seeds, 
again, are conveyed by the waters of streams and rivers, into 
which they fall ; and take root when left by the current upon a 
congenial soil. Some are even capable of resisting the influence 
of the waters of the sea; and in this manner it is that the coral 
islands, which are gradually appearing above the surface of the 



300 DISPERSION OF SEED. CONCLUSION. 

Pacific Ocean, are speedily covered with a crop of luxuriant ve- 
getation. Birds, too, are very important agents in diffusing the 
various species of plants ; some of which are scarcely dispersed in 
any other way. They carry off the whole fruit to a convenient 
place, and drop the stone when they have eaten the pulp ; or they 
eat the whole, and the seed, being undigested on account of the 
hardness of its coats, falls into the ground when voided by them. 
Some seeds will not readily germinate until they have undergone 
this process. When it is considered that from a single seed, as 
many as 30,000 or 40,000 of some species may be produced in 
a single year, it will be perceived how abundantly the Creator 
has provided for the continuance of their race, and how unlikely 
is their extinction without some great convulsion of Nature. 

472. The Reproductive System of Vegetable, then, counter- 
acts in its operation the effects which would otherwise speedily 
result from the law, which the Creator has impressed on all organ- 
ized structures; — that law of limited duration, which renders their 
death and decay as complete a portion of the series of actions 
they exhibit, as the wonderful phenomena in which they are 
concerned during life. By this counterpoise, all limit to the con- 
tinuance of races is removed, except such as is interposed by 
some causes beyond. The records of the history of the Earth, 
which are brought to light by an examination of the rocks that 
appear at its surface, afford abundant evidence that vast convul- 
sions must have formerly occurred, involving the Vegetable as 
well as the Animal kingdom ; and that, at each of these, many 
races of Plants were utterly destroyed, so that there is now pro- 
bably not a single species remaining, of those which first covered 
the dry land with verdure, when it was lifted from the depths of 
the ocean by Almighty Power. Such a convulsion will again occur. 
A time is foretold when " the elements shall melt with fervent heat, 
and the earth also and the works that are therein shall be burned 
up." But the soul of man will survive this general conflagration, 
and his faculties receive that full development of which it is of 
such vast importance that he should here become prepared. 



301 



CONCLUSION. 

The Author is not without hope, that the interest excited by 
this Treatise in the minds of some of his readers, may lead them 
to desire farther information on the subject, and to make it a 
regular object of pursuit. ■ Several works of great value to such 
inquirers exist in our own language ; among which may be spe- 
cified Dr. Lindley's "Introduction to Botany," and his Treatise 
on the subject in the " Library of Useful Knowledge ;" Professor 
Henslow's " Treatise on Botany " in Lardner's Cyclopaedia ; and 
Dr. W. B. Carpenter's "Principles of General and Comparative 
Physiology." The general plan of the following Treatise is more 
formed upon that of the last of these Volumes, than upon any 
other ; but to all of them the Author has to acknowledge his 
obligations, for the valuable materials which he has freely drawn 
from them. To those acquainted with the French and German 
languages, the " Organographie and Physiologie Vegetale" of 
Professor DeCandolle, and the " Physiologie der Pflanzen" of Pro- 
fessor Meyen, may be recommended as the most comprehensive 
works on the subject. In the Sixth Chapter, the Author has em- 
bodied a summary of the inquiries on the food of plants, recently 
made by the eminent German Chemist, Liebig ; whose Treatise 
on the " Application of Organic Chemistry to Agriculture and 
Physiology " (translated into English) he regards as one of the 
most important contributions hitherto made by Science to the 
Arts. Dr. Lindley's recent Treatise on the " Science of Horti- 
culture " may also be recommended to those who feel a desire 
for additional information on that department of the subject. 

In Schools and Families in which this Treatise is employed as 
a Class-Book, Carpenter and Westley's series of coloured paint- 
ings on glass may be exhibited with very advantageous effect, 
by means of the Magic Lantern, which is also very serviceable 
for Zoological and Astronomical illustration. 



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